In Situ Observation of (Trans-) Formation of Oxide Nanoparticles during Chemical Vapor Synthesis

Characteristics of nanoparticles (NPs) such as microstructure and morphology determine their physicochemical properties and thereby their application potential. Particle characteristics and correlated properties may be optimized by tuning relevant process parameters [1].<br/>Chemical vapor synthesis (CVS) is a dynamic gas phase synthesis to generate NPs where the formation and transformation of NPs is mainly governed by the time-temperature <i>T(t)</i> profile in the reactor [2]. Usually, the synthesized NPs are collected and characterized ex situ to determine various characteristics. This often leads to the loss of information due to artifacts during particle collection, oxidation, hydration, surface reactions or ageing of NPs. Ex situ methods also limit insight into intermediate species. Reliable information about the dynamic processes in CVS and related particle characteristics is only accessible via in situ observation.<br/>Here, we present results on CVS of tin oxide (SnO₂) and iron oxide (FeO_x) NPs investigated in situ using high energy X-rays at synchrotron beamlines [3]. The key challenge in these experiments is the low number density of the particles generated in the gas phase process which is about three orders of magnitude smaller than in corresponding bulk systems [4]. However, due to high intensity X-ray sources at contemporary synchrotron radiation facilities and modern X-ray detectors it is nevertheless possible to obtain scattering and spectroscopy data.<br/>A novel mobile CVS reactor is designed for these experiments which enables control of process parameters, especially the time-temperature <i>T(t)</i> profile and oxygen partial pressure <i>p(O₂)</i> to produce oxide NPs. In situ small and wide-angle X-ray scattering (SAXS and WAXS) are used to study microstructure, morphology, phase composition and crystal structure. X-ray absorption spectroscopy (XAS) is performed to study electronic and local structure.<br/><br/>[1] M. Winterer, Chemical Engineering Science, 186, 2018, 135-141<br/>[2] M. Winterer, Nanocrystalline Ceramics: Synthesis and Structure, Berlin: Springer 2002<br/>[3] M. A. Schroer, A. Levish, Y. Yildizlar, M. Stepponat, M. Winterer, Review of Scientific Instruments, 93, 2022, 113706<br/>[4] A. Levish and M. Winterer, In situ cell for x-ray absorption spectroscopy of low volatility compound vapors, Review of Scientific Instruments, 91, 2020, 063101<br/><br/>Acknowledgements: This research is conducted at PETRA III at DESY, Hamburg, Germany under the proposal numbers: I-20211391 at beamline P64 for EXAFS, I-20211320 and I-20220812 at beamline P62 for SAXS/WAXS and I-20220890 at beamline P21 for XRD. We gratefully acknowledge the German Research foundation (DFG) for funding through research unit FOR 2284 'Model-based scalable gas-phase synthesis of complex nanoparticles' (WI-981/14 , INST 20876/395-1 FUGG).