AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
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Observation of Incipient Particle Formation during Flame Synthesis by Tandem Differential Mobility Analysis-Mass Spectrometry (DMA-MS)
YANG WANG, Juha Kangasluoma, Michel Attoui, Jiaxi Fang, Heikki Junninen, Markku Kulmala, Tuukka Petäjä, Pratim Biswas, Washington University in St Louis
Abstract Number: 110 Working Group: Nanoparticles and Materials Synthesis
Abstract Flame synthesis of nanoparticles is widely used to produce a range of nanomaterials. Unfortunately, the incipient cluster formation by nucleation and vapor condensation is not well understood (Wang et al, 2014). This gap in our knowledge of incipient particle formation is caused by limitations in instruments, where, during measurements, the high diffusivity of sub 3 nm particles significantly affects resolution and transport loss. This work used a high resolution differential mobility analyzer (DMA) and an atmospheric pressure interface-mass spectrometer (APi-TOF) to observe incipient cluster formation during flame synthesis. By tandemly applying these two instruments, differential mobility analysis-mass spectrometry (DMA-MS) measured the size and mass of the incipient particles simultaneously, and the effective density of the sub 3 nm particles was estimated. The sub 3 nm clusters generated during the synthesis of SiO$_2 and TiO$_2 have effective densities of 1.42 g/cm$^3 and 1.75 g/cm$^3, respectively, meaning that they were less dense than the SiO$_2 and TiO$_2 bulk crystals, possibly because these particles were in an amorphous state. The APi-TOF further provided the chemical compositions of the detected clusters based on highly accurate masses and isotope distributions. Measurements in a blank flame detected a large number of sub 3 nm particles generated from chemical ionization reactions, and determined that nitrate ions dominated in the negative ions. The blank flame-generated ions played an important role during particle synthesis, since the APi-TOF observed the appearance of nitrate ions in clusters containing silicon or titanium (such as Si$_2H$_6NO$_(10)$^- and TiN$_2O$_(10)$^-). As precursor feed rate increased, particles with larger mass and sizes were formed due to coagulation and vapor condensation. The unchanged particle effective density implied that the particle formation pathway in flames was not a function of precursor feed rates.
Wang, Y. et al., (2014). J. Aerosol Sci., 71, 52-64.