Grant agreement number
POLYmer-COntrolled Mesocrystal application-oriented Production: a combined theoretical and experimental approach
Start date of the project
Duration of the project
Dr Andrew J. Sutherland
Dr Olga Boytsova
Period covered by the report
From 27/09/2015 to 27/09/2017
Mesocrystals (MCs) are a relatively new class of materials with potential in many applications e.g. photocatalysis, dye-sensitized solar cells, fibre optics, sensors, bioimplants, etc. MCs are best viewed as ordered assemblies (superstructures) of individual single crystals, each of which often have critical dimensions of the order of nanometres. Such structures are common in nature and in recent years chemists have developed routes to, and some models of, MC formation.
At the start of POLYCOMP though, approaches to mesocrystal (MC) formation were still largely ad hoc and thus many of the applications of these fascinating materials detailed above remained largely unachievable. The principle underlying reason for this is that complex MC formation processes were (and in many cases still are) too poorly understood. Based on natural crystallisation phenomena, chemists had developed a working model of MC formation whereby polymers can be used to form organised inorganic structures. This said, the shape, period, size and morphology of self-organized structures (MCs) generated in this manner show strong structural dependence upon the polymer used.
POLYCOMP sought to address this issue by focussing on a well studied system - formation of NH4TiOF3 MCs and their subsequent thermally-mediated transition into TiO2 MCs. Critically the TiO2 materials retain the MC superstructure of the precursor NH4TiOF3 MCs. POLYCOMP successfully shed further light on the initial poly(ethylene glycol) (PEG)-mediated formation process of NH4TiOF3 MCs, so adding to existing knowledge of this well studied process.
Scheme 1 Equation showing the chemical reactions involved in the formation of both NH 4 TiOF3 and TiO2 MCs from the titanium-containing precursor (NH 4 ) 2 TiF6 .
Moreover, results from POLYCOMP enabled a model to be developed explaining the origin of the effects observed when too much or too little PEG was present in the templating stage. This work was published in the second year of POLYCOMP.1
Figure 1 Images of mesocrystals of NH 4 TiOF3 (left) and TiO2 (right)
In addition to the theoretical findings outlined above, this paper also reported the photocatalytic assessment of POLYCOMP materials which proved themselves to be equally successful, in terms of photocatlytic efficiency, as other TiO2-based systems and indeed commercial ones such as Degussa P25.
In the second year of POLYCOMP focus shifted towards understanding the processes that occur in the conversion of NH4TiOF3 MCs into TiO2 MCs. Previous work had detailed the chemical processes that likely occurred in this transition but did not provide structural insights. The Fellow in conjunction with the Scientist-in-Charge co-wrote an application for beamtime at the Diamond Light Source and this visit, consolidated by a number of visits the Fellow made to the ESRF facility in France, enabled considerable progress to be made towards elucidating the structural changes that accompany each of the 4 discrete chemical transitions that occur in the thermal conversion of NH4TiOF3 MCs into TiO2.
Figure 4 XRD data and Ramen data for NH 4 TiOF3 and TiO2 mesocrystals
TiO2 is an incredibly important material for society with myriad applications. For example TiO2 is used as a photocatalyst, in solar cells to enhance energy harvesting, in energy production (by catalysing the splitting of water into hydrogen and oxygen), in sunscreens/UV pigments etc. Accordingly we believe that this second strand of work will be a lasting legacy of POLYCOMP. This work, requiring significant post sychrontron beam-time calculations, is currently being written up for publication.
Funded by the EU Framework Programme for Research and Innovation
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