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The shapes and sizes of platinum nanoparticles were controlled by changes in the ratio of the concentration of the capping polymer material to the concentration of the platinum cations used in the reductive synthesis of colloidal particles in solution at room temperature. Tetrahedral, cubic, irregular-prismatic, icosahedral, and cubo-octahedral particle shapes were observed, whose distribution was dependent on the concentration ratio of the capping polymer material to the platinum cation. Controlling the shape of platinum nanoparticles is potentially important in the field of catalysis.

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Colloidal silver sols of long-time stability are formed in the γ-irradiation of 1.0 × 10-4 M AgClO4 solutions, which also contain 0.3 M 2-propanol, 2.5 × 10-2 M N2O, and sodium citrate in various concentrations. The reduction of Ag+ in these solutions is brought about by the 1-hydroxyalkyl radical generated in the radiolysis of 2-propanol; citrate does not act as a reductant but solely as a stabilizer of the colloidal particles formed. Its concentration is varied in the range from 5.0 × 10-5 to 1.5 × 10-3 M, and the size and size distribution of the silver particles are studied by electron microscopy. At low citrate concentration, partly agglomerated large particles are formed that have many imperfections. In an intermediate range (a few 10-4 M), well-separated particles with a rather narrow size distribution and little imperfections are formed, the size slightly decreasing with increasing citrate concentration. At high citrate concentrations, large lumps of coalesced silver particles are present, due to destabilization by the high ionic strength of the solution. These findings are explained by two growth mechanisms:  condensation of small silver clusters (type-I growth), and reduction of Ag+ on silver particles via radical-to-particle electron transfer (type-II growth). The particles formed in the intermediate range of citrate concentration were studied by high-resolution electron microscopy and computer simulations. They constitute icosahedra and cuboctahedra.

Ultrafast laser spectroscopy has been used to characterize the low frequency acoustic breathing modes of Au particles, with diameters between 8 and 120 nm. It is shown that these modes are impulsively excited by the rapid heating of the particle lattice that occurs after laser excitation. This excitation mechanism is a two step process; the pump laser deposits energy into the electron distribution, and this energy is subsequently transferred to the lattice via electron–phonon coupling. The measured frequencies of the acoustic modes are inversely proportional to the particle radius; a fit to the data for the different sized particles yields v̄R=0.47cl/Rc, where R is the particle radius, cl is the longitudinal speed of sound in Au, and c is the speed of light. This functional relationship exactly matches the prediction of classical mechanics calculations for the lowest frequency radial (breathing) mode of a free, spherical particle. The inverse dependence of the frequency on the radius means that the modulations are damped for polydisperse samples. Analysis of our data shows that this inhomogeneous decay dominates the damping, even for our high quality samples (8%–10% dispersion in the size distribution). The size dependence of the electron–phonon coupling constant was also examined for these particles. The results show that, to within the signal to noise of our measurements, the electron–phonon coupling constant does not vary with size for particles with diameters between 4 and 120 nm. Furthermore, the value obtained is the same as that measured for bulk gold.

The optical absorption of the colloidal nanoparticles is investigated, formed by the UV illumination of aqueous solutions containing AgClO 4 ((1-4) × 10 -4 M), acetone (2 × 10 -2 M), 2-propanol (1 M), and various polymer stabilizers. The 7 nm particles, which are produced in the presence of polyethyleneimine, possess an unusally narrow plasmon absorption band. The wavelength and shape of this band are affected by various adsorbed solutes; adsorption phenomena can therefore be studied spectrophotometrically. The changes in band shape that occur in the presence of oxygen and of carbon tetrachloride are attributed to a partial oxidation of the silver particles by these solutes. During the oxidation, the Fermi level in the nanoparticles shifts to a more positive potential, until the oxidation comes to a halt. Chemisorbed metal cations (Cd 2+ , Ni 2+ , Ag + , Hg 2+ ) affect the plasmon absorption band of the silver nanoparticles more strongly the more electropositive is the metal. The effect is interpreted in terms of the donation of electron density from the silver particles to the adsorbed cations. In the case of Hg 2+ ions, the electron donation leads to partial Ag oxidation and amalgam formation. UV illumination of a sol that contains Cd 2+ ions produces Cd metal on the surface of the silver particles.

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