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Understanding the competing effects of growth-directing additives, such as halide ions, on particle formation in solution phase metal nanoparticle syntheses is an ongoing challenge. Further, trace halide impurities are known to have a drastic impact on particle morphology as well as reproducibility. Herein, we employ a “halide-free” platform as an analogue to commonly used halide-containing surfactants and metal precursors to isolate and study the effects of micromolar concentrations of halide ions (chloride, bromide, and iodide) on the rate of metal ion reduction. In the absence of competing halides from precursors and surfactants, we observe a catalytic effect of low concentrations of halide ions on the rate of metal ion reduction, an influence which is fundamentally different from the previously reported role of halides in metal nanoparticle growth. We propose that this halide-assisted metal ion reduction proceeds via the formation of a halide bridge which facilitates the adsorption of the metal precursor to a growing nanoparticle and, subsequently, electron transfer from the particle surface. We then demonstrate that this process is operative not only in the well-controlled “halide-free” platform, but also in syntheses involving high concentrations of halide-containing surfactants as well as metal precursors with halide ligands. Importantly, this study shows that halide-assisted metal ion reduction can be extended to bimetallic systems and provides a handle for the directed differential control of metal ion reduction in one-pot co-reduction syntheses. 

Metal nanoparticles possessing a high density of atomic steps and edge sites provide an increased population of undercoordinated surface atoms, which can enhance the catalytic activity of these materials compared to low-index faceted or bulk materials. Simply increasing reactivity, however, can lead to a concurrent increase in undesirable, non-selective side products. The incorporation of a second metal at these reactive stepped features provides an ideal avenue for finely attenuating reactivity to increase selectivity. A major challenge in synthesizing bimetallic nanomaterials with tunable surface features that are desirable for fundamental catalytic studies is a need to bridge differences in precursor reduction potentials and metal lattice parameters in structures containing both a noble metal and a non-noble metal. We report the use of low micromolar concentrations of iodide ions as a means of differentially controlling the relative reduction rates of a noble metal (palladium) and a non-noble metal (copper). The iodide in this system increases the rate of reduction of palladium ions while concurrently slowing the rate of copper ion reduction, thus providing a degree of control that is not achievable using most other reported means of tuning metal ion reduction rate. This differential control of metal ion reduction afforded by iodide ions enables access to nanoparticle growth conditions in which control of palladium nanoparticle growth by copper underpotential deposition becomes possible, leading to the generation of unique terraced bimetallic particles. Because of their bimetallic surface composition, these terraced nanoparticles exhibit increased selectivity to acetaldehyde in gas phase ethanol oxidation.