Coating thickness measurements on gas-borne nanoparticles by combined mobility and aerodynamic spectrometry
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An on-line method is described and validated to measure the thickness of coatings on gas-borne nanoparticles. The method is essentially a tandem technique which measures the aerodynamic diameter of a particle twice—before and after coating—by a single-stage low-pressure impactor (SS-LPI) for the same mobility equivalent diameter preselected via differential mobility analyzer (DMA). A shell thickness is then derived from the change in effective particle density determined by the SS-LPI. The method requires a difference in mass density between carrier particle and coating material. Its theoretical sensitivity is shown to range between about 0.1 and 1 nm, depending on the density ratio. One advantage of this approach is that both DMA and SS-LPI are situated in series but downstream of the coating step, so as not to interfere with the coating process. The method was validated against transmission electron microscopy (TEM) measurements, using spherical silica–titania particles coated with conformal shells of molybdenum and bismuth oxide by chemical vapor deposition (CVD). For such spherical particles, the agreement with TEM was excellent. The technique was able to provide layer thicknesses for sub-nanometer layers barely or not resolved by TEM. The paper also discusses the impact of ‘non-ideal’ phenomena such as the formation of doublet particles by coagulation, the effect of multiply charged particles, or the onset of homogeneous decomposition of the coating precursor. With supporting experimental data, it is shown that such phenomena can be interpreted reliably from certain features of the impactor penetration curve. The on-line method can thus be used for fast screening of process parameters and reliable process monitoring for gas-phase synthesis of composite nanopowders.
KeywordsCore–shell nanoparticles Coating thickness Effective density Aerosol characterization Inertial impaction Process monitoring
The authors want to thank Andreas Linnenbach for his great help on performing the experiments. Funding for this work was in part provided by the Deutsche Forschungsgemeinschaft (DFG) under Grant Ka-18/1373. This project is part of the JointLab IP3, a joint initiative of KIT and BASF. Financial support by the ministry of science, research and the arts of Baden-Württemberg (Az. 33-729.61-3) is gratefully acknowledged.
- Heel A (2006) Integriertes Verfahren zur Funktionalisierung der Oberfläche von gasgetragenen Partikeln durch MOCVS/MOCVD und dessen Anwendung auf die Herstellung von Pd/SiO2-Katalysatoren. PhD Thesis, Institut für Mechanische Verfahrenstechnik und Mechanik, Universität Karlsruhe (T.H.)Google Scholar
- Sigmund S, Akgün E, Meyer J, Hubbuch J, Wörner M, Kasper G (2014) Defined polymer shells on nanoparticles via a continuous aerosol-based process. J Nanopart Res 16:1–9Google Scholar