Mono- and bimetallic faceted noble metal nanoparticles : Synthesis and characterization

This study focused on creating nanoparticles of mono- and bimetallic noble metals (Ag, Au, Pd, Pt) with specific shapes and analyzing them using various methods such as spectroscopy, microscopy, and diffraction. Silver nanoparticles, approximately 35 nm in size, with a quasi-spherical shape and plasmonic behavior, were successfully synthesized via the reduction of silver nitrate by glucose in water. These nanoparticles had a five-fold twinned crystalline structure and could explain the deformation from a perfect spherical shape. Silver nanowires were synthesized through a polyol-based method involving the reduction of metal salt in the presence of a stabilizing agent, ethylene glycol (polyol method). The process allowed for anisotropic silver nanoparticles. The method utilized silver chloride seeds formed in-situ, which grew into nanowires through a seed-and-growth approach. PVP and NaCl influenced the resulting morphology by inhibiting certain facets, leading to elongated nanowires with five-fold symmetry. The anisotropic morphology of the nanowires gave rise to interesting optical properties, as observed in the UV-Vis spectrum with two maxima at 348 nm and 379 nm, corresponding to different surface plasmons. SEM images revealed uniform-width nanowires with an average width of 74 ± 15 nm and a length of 15 μm. However, determining the hydrodynamic radius through DLS was problematic due to their anisotropic shape and high length. The synthesized nanowires showed electrosteric stabilization. PXRD confirmed the phase purity and crystallinity, confirming the absence of AgCl seeds used as templates for growth. Silver nanocubes stabilized with PVP were prepared using a modified polyol process with oxidative etching. The synthesis was successfully optimized and provided nanocubes with slightly rounded corners and smooth surfaces, with an average edge length of 85 nm. Various tests showed that they were colloidally stable, with a hydrodynamic diameter of around 107 nm and a negative ζ-potential of -41 mV. They were also found to be single crystalline structures with a [001] orientation and exhibited characteristic reflections of the fcc phase of metallic silver. These synthesized nanoparticles were used for further core-shell syntheses and in-situ heating experiments.

Palladium nanoparticles were synthesized using ascorbic acid in water, resulting in 18 nm nanoparticles with well-defined shapes enclosed by (100) facets. The nanoparticles were thoroughly characterized using various methods, including UV-Vis, DLS, DCS, STEM, and PXRD, to determine their size, structure, and stability. By adjusting the amount of etching agent KBr during synthesis, a correlation was observed between bromide ion concentration and nanoparticle size, with decreasing concentrations resulting in smaller nanoparticles. The synthesis produced monodisperse, colloidal stable cubical nanoparticles, demonstrating the controllability of nanoparticle size. When using different lengths of capping agent (PVP), no significant changes in morphology and size were observed, but additional shapes, such as rods and pyramids, were generated. Through a seed-and-growth approach, the 18 nm palladium nanoparticles were used as seeds to generate larger, approximately 45 nm nanoparticles, which maintained their monodispersity and single-crystalline nature. The growth mechanism was examined by analyzing samples collected at different intervals, revealing a relatively fast growth rate in the first 7 hours of synthesis, resulting in octahedral structures. The growth rate then gradually decreased, but the nanoparticles regained their cubic shape, resulting after 24 hours to the final products. The synthesis of noble metal core-shell structures presents a challenge due to the galvanic replacement reactions that occur between metals with varying electrochemical potentials. The reaction between silver nanoparticles and gold ions could easily lead to the creation of hollow nanostructures. After careful consideration of various parameters, such as the molar ratio of the two metals, solution pH level, temperature, precursor concentration, and addition rate, a reliable and reproducible synthesis was developed. The optimal conditions resulted in the creation of 96 nm core-shell silver-gold nanocubes with a nominal composition of 75% silver and 25% gold, and a complete formation of a gold shell around the silver nanocubes. Through TEM and PXRD, it was confirmed that small, 10 nm gold nanoparticles were deposited on the surface of the silver cubes, while EDS mapping showed the precise distribution of the two metals, thereby confirming the core-shell nature of the particles. Further, TEM and PXRD experiments were conducted using Ag@Au nanocubes for in-situ heating.

Through a seed-mediated approach utilizing the galvanic replacement reaction, 100 nm platinum nanocages were successfully synthesized. The absence of LSPR peaks in the UV-Vis spectrum indicated the formation of a platinum shell. SEM and TEM imaging confirmed the complete dissolution of silver nanocubes and the formation of nanocages or other platinum nanoparticles. The platinum shell exhibited a thickness of approximately 10 nm. EDS analysis validated the platinum-rich composition, revealing a molar ratio of 95:5 for platinum to silver. TEM images further revealed that the shell consisted of 2 nm platinum nanoparticles, forming a compact frame structure. Palladium-platinum core-shell structures were also generated through a seed-and-growth method. Cubic nanoparticles with slightly rounded edges and corners, with an average edge length of approximately 21 nm, and a nominal composition of palladium to platinum 90:10 were successfully synthesized. TEM images provided evidence of the successful formation of bimetallic core-shell palladium-platinum nanostructures. The similar lattice constants of platinum and palladium allowed epitaxial deposition of platinum on the palladium seeds. However, the partial dissolution of the palladium core led to the formation of concave nanostructures. Further optimization of synthesis parameters may allow better control of the final shell thickness of the nanoparticles. In-situ heating microscopy and diffraction experiments were carried out to assess the thermal behavior of silver nanocubes and silver-gold core-shell nanocubes. The silver nanocubes exhibited stability up to 600 °C, maintaining their cubic shape, but above 600 °C, the corners rounded off, forming truncated octahedra. Above 700 °C, the silver nanocubes lost their morphology and underwent sublimation. Silver-gold core-shell nanocubes also maintained their shape up to 700 °C and underwent an alloying process above 400 °C, followed by sublimation of silver, resulting in morphological changes. XRD showed that no melting process had occurred, as the nanoparticles remained crystalline throughout the whole experiment. Ex-situ experiments on silver nanospheres showed no sublimation when heated under ambient conditions. Instead, the mass loss observed was attributed to the decomposition of the stabilizing agent, PVP.


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