Au-Cu-M (M = Pt, Pd, Ag) nanorods with enhanced catalytic efficiency by galvanic replacement reaction.

This work reports a general wet-chemistry method to produce Au-Cu-X (X = Pt, Pd, and Ag) trimetallic nanorods using galvanic replacement reaction with Au-Cu nanorods as the templates. The mild conditions, such as low temperature and slow injection of metal precursors, contributed to the slow galvanic replacement reaction and helped keep the rod structure intact. The distribution of Au, Cu and the doping metals was even in the rods as confirmed by elemental mapping. The alloyed trimetallic nanorods showed enhanced catalytic activity for p-nitrophenol reduction after incorporating the third metal. Remarkably, the Au-Cu-Pd and Au-Cu-Pt nanorods show more than an order of magnitude improvement in the mass activities compared to the Au-Cu nanorods. This facile and general synthetic method can be applied to fabricate other multimetallic nanoparticles with varying shapes and compositions.

The precipitation, growth and stability of mercury sulfide nanoparticles formed in the presence of marine dissolved organic matter.

The methylation of mercury is known to depend on the chemical forms of mercury (Hg) present in the environment and the methylating bacterial activity. In sulfidic sediments, under conditions of supersaturation with respect to metacinnabar, recent research has shown that mercury precipitates as ß-HgS(s) nanoparticles (ß-HgS(s)nano). Few studies have examined the precipitation of ß-HgS(s)nano in the presence of marine dissolved organic matter (DOM). In this work, we used dynamic light scattering (DLS) coupled with UV-Vis spectroscopy and transmission electron microscopy (TEM) to investigate the formation and fate of ß-HgS(s)nano formed in association with marine DOM extracted from the east and west of Long Island Sound, and at the shelf break of the North Atlantic Ocean, as well as with low molecular weight thiols. We found that while the ß-HgS(s)nano formed in the presence of oceanic DOM doubled in size after 5 weeks, those forming in solutions with coastal DOM did not grow over time. In addition, when the HgII : DOM ratio was varied, ß-HgS(s)nano only rapidly aggregated at high ratios (>41 µmol HgII per mg C) where the concentration of thiol groups was determined to be substantially low relative to HgII. This suggests that functional groups other than thiols could be involved in the stabilization of ß-HgS(s)nano. Furthermore, we showed that ß-HgS(s)nano forming under anoxic conditions remained stable and could therefore persist in the environment sufficiently to impact the methylation potential. Exposure of ß-HgS(s)nano to sunlit and oxic environments, however, caused rapid aggregation and sedimentation of the nanoparticles, suggesting that photo-induced changes or oxidation of organic matter adsorbed on the surface of ß-HgS(s)nano affected their stability in surface waters.

Formation of bimetallic dumbbell shaped particles with a hollow junction during galvanic replacement reaction.

The galvanic replacement reaction (GRR) has been shown to be an effective method to fine tune the structure of monometallic nanoparticles by controlling the precursor concentration and surface ligands. However, the structural evolution of nanoparticles is not well understood in multimetallic systems, where along with oxidation, dealloying and diffusion occur simultaneously. Here, we demonstrate that by controlling the rate of GRR in AuCu alloy nanorods, they can be transformed into either AuCu hollow rods or AuCu@Au core-shell spheroids. Interestingly, the transformation of rods into spheroids involved a critical intermediate state with a hollow junction and dumbbell shape. The formation of a hollow junction region was attributed to preferential diffusion of Cu atoms to the tips caused by the polycrystallinity and high curvature of the tips of the initial template. This structural transformation was also monitored in situ by single particle scattering spectroscopy. The coupling between the two ends of the dumbbell-shaped intermediate connected with a hollow metallic junction gives rise to additional plasmonic features compared with regular rods. Electrodynamic simulations showed that varying the dimensions of the hollow part by even one nanometer altered the plasmon resonance wavelength and lineshape drastically. This study shows that single particle plasmon resonance can be used as an exquisite tool to probe the internal structure of the nanoscale junctions.

An unconventional mechanism of hollow nanorod formation: asymmetric Cu diffusion in Au-Cu alloy nanorods during galvanic replacement reaction.

This study investigates how AuCu3 alloy nanorods transform into hollow rods during a galvanic replacement reaction. An unusual reaction intermediate was observed where the solid nanorod became partially hollow and Cu rich at one end. This was attributed to simultaneous galvanic replacement and asymmetric diffusion of Cu due to the Kirkendall effect. The hollow Au-Cu nanorods showed enhanced catalytic activity for p-nitrophenol reduction.

Enhanced availability of mercury bound to dissolved organic matter for methylation in marine sediments.

The forms of inorganic mercury (HgII) taken up and methylated by bacteria in sediments still remain largely unknown. From pure cultures studies, it has been suggested that dissolved organic matter (DOM) may facilitate the uptake either by acting as a shuttle molecule, transporting the HgII atom to divalent metal transporters, or by binding HgII and then being transported into the cell as a carbon source. Enhanced availability of Hg complexed to DOM has however not yet been demonstrated in natural systems. Here, we show that HgII complexed with DOM of marine origin was up to 2.7 times more available for methylation in sediments than HgII added as a dissolved inorganic complex (HgII(aq)). We argue that the DOM used to complex HgII directly facilitated the bacterial uptake of HgII whereas the inorganic dissolved HgII-complex adsorbed to the sediment matrix before forming bioavailable dissolved HgII complexes. We further demonstrate that differences in net methylation in sediments with high and low organic carbon content may be explained by differences in the availability of carbon to stimulate the activity of Hg methylating bacteria rather than, as previously proposed, be due to differences in HgII binding capacities between sediments.

Structural defect induced peak splitting in gold-copper bimetallic nanorods during growth by single particle spectroscopy.

A single particle level study of bimetallic nanoparticle growth provides valuable information that is usually hidden in ensemble measurements, helping to improve the understanding of a reaction mechanism and overcome the synthetic challenges. In this study, we use single particle spectroscopy to monitor the changes in the scattering spectra of Au-Cu alloy nanorods during growth. We found that the unique features of the single particle scattering spectra were due to atomic level geometric defects in the nanorods. Electrodynamics simulations have demonstrated that small structural defects of a few atomic layers split the scattering peaks, giving rise to higher order modes, which do not exist in defect-free rods of similar geometry. The study shows that single particle scattering technique is as sensitive as high-resolution electron microscopy in revealing atomic level structural defects.