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

Abstract

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.

Measuring ultrafine aerosols by direct photoionization and charge capture in continuous flow

“…The experimental and model results agree well for a range of particle mobility diameters (25-80 nm) (25-65 nm) with agreement up to 80 nm and concentrations (9 £ 10 4 to 1.1 £ 10 6 cm ¡ 3 ) using a single value for K c I. The results show that the mobility diameter can be used as the characteristic particle diameter, d p , in Equation (1) even for aggregates (Keller et al 2001;Burtscher 2005;Zhou et al 2013), agreeing with previous research for the range of soot morphology measured in this work (Stettler et al 2013). For this reason, the CMD for mobility diameter is represented by d p in the remaining discussion.…”

“…Experimental results show photoelectric yield to be a linear function of inverse electrical mobility for carbon agglomerates (Burtscher 1992), fractal-like silver agglomerates (Schmidt-Ott et al 1990) (20 < d m < 100 nm, d fm D 2.26) (Schmidt-Ott 1988;Wang and Sorensen 1999), diesel soot (Burtscher et al 1998), and recrystallized Ag and Au (Keller et al 2001), for particle mobilities largely in the free-molecular regime (20 < d < 100 nm). Photoemission yield is found to be a linear function of total particle (mobility) surface area, a nearly equivalent measurement to total active surface, for polyaromatic hydrocarbon (PAH) coated graphite aerosols (measured by diffusion battery) (Niessner 1986), denuded diesel exhaust particles at five representative engine modes (measured by scanning mobility particle sizer, SMPS) (Kittelson et al 2005), and sintered spheres and fractal-like agglomerates of Ag (SMPS) (Zhou et al 2013). Measurements of the aerosol total active surface area are of interest for emissions, environmental, or exposure monitoring, particularly for materials such as elemental carbon and diesel soot, however, the dependence of particle material and morphology on photoemission must be understood.…”

Measuring aerosol active surface area by direct ultraviolet photoionization and charge capture in continuous flow

“…In the free molecular regime, the inverse of particle mobility is largely proportional to the square of mobility diameter, d 2 m . The dependence of photoemission yield on the inverse of particle mobility has been demonstrated in the free molecular and transition regimes (20 < d p < 100 nm) for sintered spheres and fractal-like silver agglomerates (Schmidt-Ott et al, 1990;Schmidt-Ott, 1988;Zhou et al, 2013), recrystallized Ag and Au (Keller et al, 2001), poly-aromatic hydrocarbon (PAH) coated graphite aerosols (measured by diffusion battery) (Niessner, 1986), carbon agglomerates (Burtscher, 1992;Nishida et al, 2018), and diesel soot (Burtscher et al, 1998;Kittelson et al, 2005).…”

“…The photoelectric quantum yield has also been found to vary between different morphologies of the same homogeneous material type. Three studies which discuss morphology effects on aerosol photoemission in detail seemingly contradict one another (Zhou et al, 2013;Keller et al, 2001;Schmidt-Ott, 1988). Zhou et al (2013) found the overall charging efficiency of silver aggregates to be 0.4-0.8 times that of silver spheres of the same mobility diameter.…”

“…Three studies which discuss morphology effects on aerosol photoemission in detail seemingly contradict one another (Zhou et al, 2013;Keller et al, 2001;Schmidt-Ott, 1988). Zhou et al (2013) found the overall charging efficiency of silver aggregates to be 0.4-0.8 times that of silver spheres of the same mobility diameter. This result disagrees with an existing photocharging theory which suggests that particles of the same material type and mobility should have the same photoelectric charging efficiency regardless of their morphology (Keller et al, 2001).…”