Fine metal particles and fine metal oxide particles in dry powder form, and use thereof

A process for preparing fine metal particles in the form of a dry powder, characterized in that

• the fine metal particles in the form of a dry powder are free from any solvent,
• an average particle size of the fine metal particles themselves are selected in the range of 1 to 20 nm,
• the surface of the fine metal particles themselves is covered with one or more carboxylic acids selected from the group consisting of long chain carboxylic acids having 8 or more carbon atoms in the form of linear carboxylic acid, which carbon atoms are chosen in the range of 8 to 18 carbon atoms,
• a covering amount of said one or more carboxylic acids is adjusted by selecting the total of said one or more carboxylic acids in the range of 5 to 35 parts by mass based on 100 parts by mass of the fine metal particles themselves, and thus, the fine metal particles in the form of a dry powder are composed of the fine metal particles themselves and the one or more carboxylic acids;
• and
• as for the step of adjustment of the covering amount, the process comprises the steps of:
• beforehand bringing said one or more compounds into contact with the surface of the fine metal particles themselves having an average particle size selected in the range of 1 to 20 nm, thereby once applying said one or more compounds through a coordinative bond with a metal element contained in the fine metal particles in an amount greater than the aimed covering amount in total of said one or more compounds based on 100 parts by mass of the fine metal particles themselves to form a covering layer thereof, and preparing, as a starting material, a dispersion in which the fine metal particles having a covering layer of said one or more compounds are dispersed in a dispersion solvent comprising one or more organic solvents,
• removing the organic solvent contained in the dispersion as a dispersion solvent under reduced pressure, thereby concentrating the dispersion,
• adding, to the dispersion subjected to the treatment for concentration, one or more polar solvents in which said one or more compounds exhibit a higher solubility at room temperature than that in the organic solvent, thereby dissolving excess of one or more compounds in said one or more polar solvents, and then separating fine metal particles in which the adjustment of the covering amount is attained by removing the excess of one or more compounds, as a solid phase component, from the obtained dispersion by filtration, and
• performing the treatment of evaporating the remaining one or more polar solvents at a temperature of 100° C. or lower to dry up,
• wherein
• the fine metal particles themselves are fine metal particles of a metal species selected from the group consisting of gold, silver, copper, platinum, palladium, tin, nickel, aluminum, zirconium, titanium, iron and tungsten, or fine alloy particles comprising two or more metals selected from the metal species group,
• a thickness of the covering layer formed with the adjusted covering amount is at least 0.5 nm or thicker, and selected in the range of 2/10 to 8/10 of the average particle size of the fine metal particles, and
• the one or more carboxylic acids are fixed by a coulombic interaction to the metal surface of the fine metal particles themselves to form the covering layer.