Bipolar electrospray for the synthesis of defined hetero-aggregates: Process design and in situ diagnostics

Project Leader :

Prof. Dr. rer. nat. Alfred Weber
Clausthal University of Technology

Prof. Dr.-Ing.Stefan Will
Friedrich-Alexander-Universität Erlangen-Nürnberg

Aggregates of two different particle materials in contact with each other offer a variety of applications ranging from energy supply and environmental remediation to improved biosensors. However, a fundamental understanding of contact formation and its relationship to the specific powder application has not yet been achieved. The missing link is the characterization of the contact properties and the correlation with the functional particle properties. The main objective of the second phase of this project is to carry out the synthesis of different particle systems for energy and environmental technology using the already successfully implemented bipolar electrospray (BP-ES) and to comprehensively characterize the resulting particles and their functionality using novel optical methods and on/offline analytics. This first requires an optimization of the BP-ES for the synthesis of the respective particle system as well as the development of reactors for the investigation of the particle functionality. In order to be able to correlate the functionality of the particles with the properties of the heterocontact and the synthesis parameters, further methods for process analysis are being further developed and optimized for use on heteroparticles in addition to established methods.

In this project, heteroparticle systems are synthesized and investigated for three areas of application. On the one hand, these are metal/metal oxide particles such as Ag/TiO₂ for photocatalytic dye decomposition in the field of environmental remediation, for which a compact photocatalysis reactor is being optimized for the still low production rates of ES. For the energy sector, another CeO2/TiO₂ particle system is being used for the photocatalytic production of H₂, for which a reactor is also being developed to capture the quantities of H₂ produced. Finally, in the field of environmental remediation, TiO₂ particles on flake-like particles of graphene oxide or boron nitride are being investigated for the decomposition of PFAS, whereby in particular the formation of CO₂ and the change in pH value over time due to dissolved hydrogen fluoride are to be used as indicators for the decomposition rate.

For correlation with synthesis parameters, the BP-ES itself must first be characterized in terms of droplet evaporation kinetics and mixing efficiency, which is done with a combination of WALS for droplet size measurement and a PIV-like setup to determine the spray shape and droplet velocity. An absorption-emission technique is used to determine the mixing efficiency of the charged precursor/solvent droplets. The absorption of laser light for certain dyes is greatly reduced after mixing compared to unmixed droplets and thus fluorescence is also suppressed (LIF-AS). The elemental composition of the heteroparticles, which is investigated using laser-induced breakdown spectroscopy (LIBS), is also influenced by the mixing efficiency. The size and shape of the particles are determined using wide-angle light scattering (WALS), with a focus on the development of suitable scattering models for flake-like particles. The absorption properties of the synthesized particles, which significantly influence the photocatalytic activity, are investigated using UV-Vis absorption spectroscopy. In addition, other online and offline techniques, such as aerosol photoemission spectroscopy, TEM, XPS and ICP-MS, are used to verify the optical techniques.

Selected cooperations:

• Prof. Martin Sommerfeld (OvG University Magdeburg): Numerical modeling of droplet collisions and the particle formation process
• Prof. Volker Schmidt (University of Ulm): Use of advanced automated routines for the evaluation of scattered light spectra
• Prof. Andreas Bück / Dr. Jochen Schmidt (FAU Erlangen-Nuremberg) WALS application / photocatalysis
• Dr. Jean-Pascal Borra (University of Paris-Saclay): Characterization of the stability ranges of EHDA sprays