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.

Colloidal gold nanoparticles with an average radius of 15 nm have a surface plasmon absorption band at 530 nm. Excitation by laser pulses of 450 fs duration, and wavelength of 600 or 380 nm “bleached” the plasmon band and produced a transient absorption at the wings of the “bleach” spectrum. The transient absorption was found to have a similar temporal behavior at different wavelengths. Analysis of their temporal behavior showed two time constants:  2.5 ps, and a slower component of >50 ps. Laser excitation close to the plasmon band at 600 nm leads to the formation of “hot” non-Fermi electronic distribution within the colloidal particles. Transient absorption from these “hot” electrons led to different absorptions from that of the plasmon absorption of “cold” electrons. The “hot” electrons relax via electron−phonon coupling in 2.5 ps, and the phonon−phonon relaxation of the lattice occurs in >50 ps. At 380 nm excitation, the amplitude of the blue wing becomes smaller, and the slow component becomes longer, which could be due to possible excitation of the d-band electrons. These results are discussed in terms of Mie theory and a two-temperature model (TTM), and their consequences on the optical absorption spectrum.

The electronic dynamics of gold nanocrystals, passivated by a monolayer of alkylthiol(ate) groups, were studied by transient spectroscopy after excitation with subpicosecond laser pulses. Three solution-phase gold samples with average particle size of 1.9, 2.6, and 3.2 nm with size distribution less than 10% were used. The photoexcitation in the intraband (surface plasmon region) leads to the heating of the conduction electron gas and its subsequent thermalization through electron-electron and electron-phonon interaction. The results are analyzed in terms of the contribution of the equilibrated "hot" electrons to the surface plasmon resonance of gold. A different spectral response was observed for different sizes of gold nanoparticles. The results were compared to the dynamics of the large (30 nm diameter) gold nanocrystals colloidal solution. The size-dependent spectral changes are attributed to the reduction of the density of states for small nanoparticles. The observed variation in the kinetics of the cooling process in gold nanoparticles with changing the laser intensity is attributed to the temperature dependence of the heat capacity of the electron gas.

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Growth of gold nanorods (AuNRs) by photochemical reduction of HAuCl4 in a micelle solution of hexadecyltrimethylammonium bromide (CTAB) and tetraoctylammonium bromide (TOAB) is studied. The effects of 300 and 254 nm UV light sources and their photon flux on the anisotropic growth of gold nanoparticles are investigated by controlling duration of irradiation and the number of lamps within a photochemical reactor. The resulting AuNRs were characterized by absorption spectroscopy, FTIR, and TEM. Experimental results indicate that a higher density of longer colloidal AuNRs form by increasing the number of incident photons (lamps) at 300 nm while the 254 nm lights produce a lower yield of shorter AuNRs. The yield of AuNRs also depends on the duration of irradiation which was found to be 6.00 h for 300 nm and 5.00 h for 254 nm radiation. Acetone is found to play a major role in the synthesis of AuNRs. Two mechanisms are proposed for the synthesis of Au nanoparticles in the presence and absence of acetone. Irradiation of samples for an excess time produces a lower concentration of AuNRs and a higher yield of spherical particles. This effect is attributed to atom-by-atom dissolution of AuNRs into Au-spherical particles.

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Growth of gold nanorods is investigated in the presence of ascorbic acid in an aqueous mixed micelle solution of hexadecyltrimethyl ammonium bromide (CTAB) and tetraoctyl ammonium bromide (TOAB) through photochemical reduction of HAuCl4·3H2O using 300 nm Hg lamps. The role of the ascorbic acid concentration, [H2A], in the formation of gold nanorods was studied using UV−vis absorption and transmission electron microscopy. Gold nanorods were produced only if [H2A]/[Au3+] ≥ 0.75 in the reaction cell. For 0.75 < [H2A]/[Au3+] < 6.2, the aspect ratio of nanorods increased as [H2A] increased. For [H2A]/[Au3+] > 6.2, the aspect ratio decreased, and shorter nanorods formed as [H2A] increased. These results are indicative of a lower and upper limit in the concentrations of ascorbic acid and its conjugate base (ascorbate anion) for the formation of gold nanorods. The roles of ascorbic acid and ascorbate ions are discussed, and these observations support the existence of soft templates formed by CTAB/TOAB micelles where Au nanorods grow. Evidence for parallel nucleation and growth processes is also presented.

The photodissociation dynamics of ICl is studied at 304 nm by state-selective photofragment translational spectroscopy. Velocity distributions, anisotropy parameters, and relative quantum yields are obtained for the ground I(2P3/2) and spin−orbit excited state I*(2P1/2) iodine atoms, which are produced from photodissociation of ICl at this wavelength. Two sharp velocity distributions are observed for the I channel, suggesting the existence of two dissociation pathways that correlate with ground state iodine formation. Based on the expected translational energy release and the energy separation between those peaks, the two distributions are assigned to dissociation of ICl to I(2P3/2) + Cl(2P3/2) and to I(2P3/2) + Cl*(2P1/2); the former channel appears at higher translational energy. The distribution of I* also shows two strong peaks, indicating that there are two dominant channels for the formation of I* atoms at this wavelength which we assign to the dissociation of ICl forming I*(2P1/2) + Cl(2P3/2) and I*(2P1/2) + Cl*(2P1/2), respectively. The quantum yield of I*(2P1/2) is determined to be 0.30, indicating that the formation of ground state iodine is clearly the favored dissociation channel at 304 nm. The observed anisotropy in the angular distribution of dissociation products (β) indicates that the I* + Cl and I* + Cl* channels are formed predominantly from the parallel transition (β = 1.7 for both channels) while the I + Cl and I + Cl* products are formed mainly from perpendicular transitions (β = −0.5). The decrease in the anisotropy parameter of the I formation channels from their limiting value of −1 is attributed to the presence of more than one path for the formation of I + Cl/Cl* photoproducts with opposite polarization for their absorbing transitions. The possible excited state dynamics, which give the observed results, are discussed in terms of the previously proposed energy correlation diagram for ICl.