Wavelength-Resolved UV Photoelectric Charging Dynamics of Nanoparticles: Comparison of Spheres and Aggregates

The objective of this work is to understand the charging dynamics of metal nanoparticles under wavelength-selected UV irradiation, with a particular focus on the effect of particle structure on the quantum yield. We employed an ion mobility analysis technique to measure the size-resolved single charging efficiency of structure-controlled silver nanoparticles (spheres vs. aggregates) in the mobility diameter (D m ) range of 10 ∼ 100 nm. We found that the measured particle charging efficiency follows D 2 m dependence for both spherical and aggregate particles. Based on the measured charging efficiency and calculated particle photon absorption cross section, we are also able to determine the mobility size dependence of photoelectric quantum yield for both spheres and aggregates. The quantum yield of spheres is a constant for larger particles (50 nm or larger) but significantly enhanced as particle size decreases. The quantum yield of aggregates is shown to be particle structure dependent and does not behave as a simple summation of individual primary particles. The aggregate particles have higher quantum yield compared with spheres of the same mobility size but is offset by the lower photon absorption cross section, and thus overall charging efficiency of aggregates is lower than spheres of the same mobility size. © 2013 American Association for Aerosol Research