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X-ray absorption spectroscopy (XAS) provides a unique, atom-specific tool to probe the electronic structure of solids. By surmounting long-held limitations of powder-based XAS using a dynamically averaged powder in a Resonant Acoustic Mixer (RAM), we demonstrate how time-resolved in situ (TRIS) XAS provides unprecedented detail of mechanochemical synthesis. The use of a custom-designed dispersive XAS (DXAS) setup allows us to increase the time resolution over existing fluorescence measurements from ∼15 min to 2 s for a complete absorption spectrum. Hence, we here establish TRIS-XAS as a viable method for studying mechanochemical reactions and sampling reaction kinetics. The generality of our approach is demonstrated through RAM-induced (i) bottom-up Au nanoparticle mechanosynthesis and (ii) the synthesis of a prototypical metal organic framework, ZIF-8. Moreover, we demonstrate that our approach also works with the addition of a stainless steel milling ball, opening the door to using TRIS-DXAS for following conventional ball milling reactions. We expect that our TRIS-DXAS approach will become an essential part of the mechanochemical tool box.
Time resolved in situ monitoring of mechanochemical transformations by X-ray spectroscopy (XAS)
(2021)
Mechanochemical reactions promise a new direction for environmentally benign preparation of materials, and has been dubbed by IUPAC as one of the 10 chemical innovations that will change our world. Despite this significant promise, very little is known about the mechanisms that drive mechanochemical transformations, posing significant barriers to realizing their full potential. To this end, there is growing need to follow mechanochemical reactions in situ and in real time. We here describe advances in the development and application of XAS methods to monitor material synthesis in real time under mechanochemical conditions. We demonstrate the generality of our approaches by describing mechanochemical syntheses of materials by both vibratory ball milling and by Resonant Acoustic Mixing (RAM), where a time resolution of 1 second is for a whole XAS spectrum was achieved. Moreover, we describe how spectroscopic methods can be coupled to diffraction-based approaches, thereby providing new dimensions in understanding mechanochemical synthesis.
Time-resolved WAXS studies on the formation and dissolution of polynuclear aluminium sulfates
(2016)
Polynuclear aluminium species (Al13) find application in different areas like water purification, contaminant transport, and as pilling clays with high specific surface areas, due to their strong binding ability to aggregates and high positive charge.
In the present contribution, we report on the in situ investigation of the Al13 sulfate synthesis and dissolution by synchrotron wide-angle X-ray scattering (WAXS). Al13 sulfates were crystallized by precipitating hydrolyzed aluminum solutions by the addition of sodium sulfate. Dissolution of the formed Al13 species was carried out by adding hydrochloric acid. The measurements were performed using a custom-made acoustic levitator as sample holder. The study provides information about the intermediates during the crystallization and dissolution processes. From the data, a mechanism was derived indicating the influence of the crystallization process.
Time-resolved studies on the formation of maghemite nanoparticles combining fast-XANES and SAXS
(2016)
Iron oxide nanoparticles find application in different areas like sensing, magnetic storage media, and biomedicine, due to their magnetic properties and environment-friendliness.
In the present contribution, we report on the in situ investigation of an iron oxide nanoparticle synthesis by coupled X-ray absorption near-edge structure (XANES) and small-angle X-ray scattering (SAXS). The combination provides simultaneously information about the size of particles (SAXS) and on the oxidation state and the local structure of the iron atoms (XANES). The co-precipitation synthesis was exemplary studied, using a stabilization agent to decelerate the fast precipitation of the iron oxides. This allows to detect intermediates in situ. The measurements were performed using a custom-made acoustic levitator as sample holder. From the data, a mechanism was derived indicating different phases of particle Formation and oxidation state changes.