Title : In situ Raman spectroscopic monitoring of mechanochemical preparations of energy-related materials
Mechanochemical reactions by ball milling are becoming increasingly popular across a wide range of chemical sciences, but understanding and evaluation of the processes included in these processes remain a persistent challenge. It is important to note that ex-situ measurements can give a distorted picture on the investigated system, since temporary stopping of the reaction and opening of the milling jar can significantly change the reaction pathway. Thus, it was important to develop the techniques able to give an insight into the mechanochemical reactions in-situ and in real time. Obviously, the understanding of mechanochemical processes, especially the detailed understanding of mechanistic details, is crucial for optimization of these reactions. Recently, a few techniques that allow in-situ monitoring of ball milling reaction in real time are developed and well established. For now, they include in-situ XRD and Raman spectroscopic monitoring, as well as follow-up of the temperature inside the milling jar during reaction. Here we concentrate only on Raman spectroscopic monitoring, since exclusively this technique was applied to synthesis energy-related materials.
For the first time, we applied one of these techniques, Raman spectroscopy, in order to follow up the mechanochemical synthesis of materials for solid-state hydrogen storage. More specifically, the technique was applied to a series single- and bimetallic amidoboranes of MAB, MAB•AB, M2MgAB4 and M2CaAB4 (M = Li, Na ; AB = NH3BH3) composition, that were prepared and thoroughly characterized. A detailed analysis of spectral data revealed that a common two-step mechanism takes place through MAB•AB adducts as a key intermediate phase.
In addition, the technique was applied to mechanochemical preparation and exfoliation of the systems based on 2D transition metal dichalcogenides of the TX2 composition (T = Mo, W; X = S, Se, Te), as well as exfoliation of TO3 type transition metal oxides and graphene by use of widely accessible exfoliants, such as oxalic acid, sugars, ammonia borane etc.
Our results show that Raman spectroscopic monitoring of ball milling reactions undoubtedly provides a unique insight into the mechanochemical reactions under run, in real time.