Fumitaka Mafuné

Single laser pulse induced aggregation of gold nanoparticles

Gold nanoparticles with an average diameter of 11 nm (Au(39000)) were prepared in an SDS aqueous solution. A 80-microm liquid droplet (microdroplet) of the solution was ejected into the atmosphere from a microdroplet nozzle. Structural changes of the gold nanoparticles in the microdroplet, after they were irradiated with a focused single-nanosecond laser pulse at the wavelength of 532 nm, were studied by transmission electron microscopy (TEM) and optical absorption spectroscopy. It was revealed that the gold nanoparticles are fragmented into small particles and then the small fragments aggregate with each other. The aggregation was found to be terminated 100 micros after the laser-pulse excitation.

Ionization Energies of Niobium Carbide Clusters NbnCm (n = 3−10, m = 0−7)

We prepared niobium carbide clusters, Nb(n)C(m), in the gas phase by a double laser ablation technique. A photoionization efficiency was measured as a function of the wavelength of an ionization laser to determine ionization energies (E(i)s) of Nb(n)C(m) (n = 3-10, m = 0-7). The E(i)s of Nb(4)C(4) and Nb(5)C(3) are found to be the lowest in the clusters studied. When the experimental E(i)s are compared with the E(i)s estimated by the density functional calculation previously reported (Harris, H.; Dance, I. J. Phys. Chem. A 2001, 105, 3340-3358), we determined Nb(5)C(3) to have a similar 2 x 2 x 2 cubic structure as Nb(4)C(4). We also found that the E(i)s of carbon-rich clusters, Nb(n)C(m) (n < or = m), tend to be higher (> 5 eV) than the E(i)s of the niobium-rich clusters. The high E(i)s are due to the structure of the carbon-rich clusters: carbon-carbon bonding is preferred when the number of carbon atoms exceeds the number of metal atoms. The structure was also discussed in relation to the reactivity of cluster ions with a hydrogen molecule.

Uptake of gas-phase nitrous acid by pH-controlled aqueous solution studied by a wetted wall flow tube.

Uptake kinetics of gas phase nitrous acid (HONO) by a pH-controlled aqueous solution was investigated by using a wetted wall flow tube. The gas phase concentration of HONO after exposure to the aqueous solution was measured selectively by the chemical ionization mass spectrometer in a high sensitive manner. The uptake rate of the gaseous HONO was found to depend on the pH of the solution. For the uptake by neutral and alkaline solutions, the gas phase concentration was observed to decay exponentially, suggesting that the uptake was fully limited by the gas phase diffusion. On the other hand, the uptake by the acidic solution was found to be determined by both the gas phase diffusion and the liquid phase processes such as physical absorption and reversible acid dissociation reaction. The decay was analyzed by the rate equations using the time dependent uptake coefficient involving the saturation of the liquid surface. While the uptake processes by the solution at pH = 2-3 were well described by those calculated using the physical and chemical parameters reported for the bulk, the uptake rates by the solution at 4 < pH < 7 deviate from the calculated ones. The present result can suggest that the pH at the liquid surface is lower than that in the bulk liquid, which is responsible for the additional resistance of mass transfer from the gas to the liquid phase.

In Situ Measurements of Atmospheric Nitrous Acid by Chemical Ionization Mass Spectrometry Using Chloride Ion Transfer Reactions

Recently, chemical ionization mass spectrometry (CIMS) has been widely applied to the in situ measurements of atmospheric trace species. In this article, we propose a new chemical ionization scheme using a chloride ion transfer reaction from SO(2)Cl(-) as the reagent ion and discuss the applicability of this technique to the detection of nitrous acid (HONO) in the atmosphere. From laboratory investigations, the detection sensitivity was found to depend on the flow rate of SO(2) introduced into the ion source region and the pressure inside the chemical ionization region, which suggests that the chemical ionization reaction is reversible. The detection sensitivity was well described in terms of the forward and backward rates. The present limit of detection is estimated to be 60 parts per trillion by volume (pptv) for an integration time of 1 min. Improvement of the CIMS instrument would enable the measurements of the daytime level of HONO, which might be less than 50 pptv. In addition, the possibility of the interference is discussed from thermodynamic considerations based on ab initio calculations, and the effects of the sampling artifacts are experimentally quantified.

Breakdown of the Hume-Rothery Rules in Sub-Nanometer-Sized Ta-Containing Bimetallic Small Clusters

The Hume-Rothery rules are empirical rules to predict the solid solubility of metals. We examined whether the rules hold for sub-nanometer-sized small particles. We prepared bimetallic cluster ions in the gas phase by a double laser ablation technique. Taking advantage of the magic compositions of the bimetallic cluster ions relating to the distinguished stabilities, the coalescence or the segregation of Ta and another element in the sub-nanometer-sized clusters was discussed. It was found that W, Nb, and Mo readily coalesce with Ta, while Ag, Al, Au, Co, Cu, Fe, Hf, Ni, Pt, Ti, and V are segregated from Ta. On the basis of these results, we concluded that the Hume-Rothery rules do not hold for sub-nanometer-sized particles.

Laser Fabrication of Nanomaterials in Solution

We developed new methods to form small nanomaterials in a solution, which include a liquid-phase laser ablation and laser-induced structural change of the nanomaterials. Here, we report our recent advances of the methods on gold as the typical example, and on manganese as an application.