Synthesis and chemical reactivity of polyol prepared monodisperse nickel powders

Monodisperse 1350 Å Ni powders are prepared according to the polyol process using polyvinylpyrrolidone (PVP) as a protective agent. Smaller size monodisperse Ni powders (300 Å) have presently been made from Ni(OH)2 in ethylene glycol (EG) and PVP using Pd or Pt as nucleating agents. Thermogravimetric Analysis (TGA) and Temperature Programmed Desorption (TPD) measurements in air showed that ethylene glycol chemisorbed on the Ni surface protects the fine particles from air oxidation up to 180°C. The oxidation peak temperature is reduced from 650°C for Ni of 1 to 2 μm to 260°C for Ni of 300 Å. The unseeded pure Ni powder undergoes stoichiometric oxidation to become NiO with a 27% weight gain while the oxygen uptake is 18.6% for 300 Å Ni particles seeded with 1% Pd. In that case, the formation of a new (Ni1-xPdxO1-y) type oxide crystallizing in a tetragonal structure similar to PdO is suggested. While the Ni particles of 1350 Å mean size followed a parabolic oxidation law (% oxide formed∝t1/2), 300 ÄNi particles followed a cubic law (% oxide∝t1/3). For Ni seeded powders with 1% Pt, the oxygen uptake is only 15.7%; the remaining Ni not undergoing oxidation is an Ni1-xPtx alloy phase. A model calculation shows that 300 Å Ni particles grow around 70 Å Pd or 60 A Pt nuclei and that the particles are covered by half an EG monolayer

Electrochemical reduction of noble metal species in ethylene glycol at platinum and glassy carbon rotating disk electrodes

Linear sweep voltammetry has been used to delineate the electrochemical behavior of ethylene glycol, and to determine the reduction potential of several noble metal species in this solvent at room temperature. Ethylene glycol was found to be electrochemically inactive between - 1.15 and 1.65 V at a glassy carbon electrode, and between - 0.82 and 2.0 V at a Pt electrode. Metal reduction potentials determined using both rotating electrodes follow the sequence: AuCl4 - > Ag+ > PtCl6 2- > Pd(NH3)4 2+. Under all conditions tested, ethylene glycol oxidation began at potentials more positive than metal reduction ones, thus suggesting that ethylene glycol cannot reduce these noble metal species. However, finely divided Ag and Au, were synthesized at room temperature by reduction of their corresponding ions with ethylene glycol (the basis of the polyol process). This observed difference between electrochemical results and chemical synthesis can be explained by recognizing that measured potentials are the sum of a thermodynamic potential and overpotential. Comparison between metal reduction potentials and temperature for metal particle synthesis indicates that the potential becomes more negative as the temperature increases. These results may provide useful information to better understand the fundamentals of the polyol process

SYNTHESIS OF MONODISPERSE Au, Pt, Pd, Ru AND Ir NANOPARTICLES IN ETHYLENE GLYCOL

Abstract-Au, Pt, Pd, Ru and Ir nanoparticles with a narrow size distribution have been synthesized by chemical reduction of their corresponding metal species in ethylene glycol. In all cases, the average particle size was found to be smaller than 10 nm. Particle size was mainly controlled by varying the initial total metal concentration, the reaction temperature, and the concentration of PVP. With the exception of Ir, metal particle agglomeration and sintering was prevented by the addition of PVP, a well known protective agent that also aids particle dispersion

Electrochemical reduction of noble metal compounds in ethylene glycol

The effect of temperature on both the electrochemical oxidation of pure ethylene glycol and the reduction of AuCl4 - in ethylene glycol at a rotating disk glassy carbon electrode has been investigated using linear sweep voltammetry. As the temperature is increased from 25°C up to 60°C, ethylene glycol begins to oxidize at lower potentials, whereas the reduction potential of AuCl4 - is independent of temperature. Reduction current densities, however, increase as temperature increases. Room temperature reduction of several noble metal species in ethylene glycol was also investigated. Metal reduction potentials at both a platinum and a glassy carbon electrodes follow the sequence: AuC14 ->Ag+>PtC16 2->Pd(NH3)4 2+. The oxidation potential of ethylene glycol at both electrodes was found to be more positive than the reduction potential of the gold, silver, platinum and palladium precursors. These results predict that the spontaneous formation of noble metal particles by chemical reduction with ethylene glycol is thermodynamically unfavorable at 25°C. Gold and silver particles, however, are easily prepared at room temperature using the polyol process, which is a redox based process for the preparation of finely divided metals by chemical reduction of the corresponding metal precursors with ethylene glycol. Since measured potentials are the sum of a thermodynamic and a kinetic contribution (the overpotential), metal reduction in the polyol process seems to be aided by the overpotential. Therefore, measured potentials have been correlated to the chemical conditions at which noble metal particles are synthesized in the polyol process. It was found that as the potential difference between ethylene glycol oxidation and metal reduction increases, both the reaction temperature and time needed for metal synthesis increases. These electrochemical results may contribute to have a better understanding of the fundamentals of the polyol process, and for optimizing such reaction parameters as temperature, time and solution chemistry. © Elsevier Science Ltd

Pd and Pt nucleated nano Ni-particles: Evidence for heterogeneous nucleation and growth by transmission electron microscopy

Transmission electron microscopy (TEM) study of mono-dispersed Ni-particles, prepared by the reduction of Ni(OH)2 in ethylene glycol with Pd/Pt as nucleating agent, has been carried out. Microstructural analysis employing bright-field and dark-field electron microscopy reveals polymicrocrystalline nature of each Ni-particle. TEM studies of Ni particles show presence of nearly 20 Å thick NiO layer over each 300 Å Ni-particles. Presence of 40 to 60 Å Pd/Pt nucleus has been confirmed in high magnification bright-field micrographs. The observed radial growth morphology supports heterogeneous nucleation of Pd/Pt over which Ni grows