First measurement of gas adsorption to free ultrafine particles:O2on Ag

Abstract

Gas adsorption to particles 7 nm in radius and free from contact with any substrate is measured for the first time. The particles are suspended in He doped with an adsorbing gas of concentration c and the photothreshold change is measured as a function of c. The He does not interfere with adsorption kinetics. For ${\mathrm{O}}_2$ on Ag at 300 K the sticking coefficient α is determined as a function of the coverage θ. In contrast to findings on macroscopic surfaces, the small-particle result is α=${\alpha }_0$(1-θ${)}^2$, indicating simple dissociative adsorption, and ${\alpha }_0$ is very small compared to flat-surface data.