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POLYmer-COntrolled Mesocrystal application-oriented Production: a combined theoretical and experimental approach.

Periodic Reporting for period 1 - POLYCOMP (POLYmer-COntrolled Mesocrystal application-oriented Production: a combined theoretical and experimental approach.)

Reporting period: 2015-09-28 to 2017-09-27

Mesocrystals (MCs) are a relatively new class of materials with superb 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. As its overall objective 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. A better understanding of how to form MCs is important for society because of the long term promise they offer. This potential is based on the fact that although they are micro/macroscopic materials they have the potential to possess the properties of their constituent nano-sized building blocks. Such properties include unique light emitting properties (cf quantum dots ), superparamagnetism (cf Fe3O4 nanoparticles ) etc. Consequently, there are myriad potential high tech applications possible including in: photocatalysis, Li-ion battery and electrode applications, photovoltaics (especially generation3 cells), sensors, low energy lighting systems, etc.
POLYCOMP had four primary objectives:

O1 To modify/synthesise a series of polymers with the potential to control MC formation.
O2 To evaluate polymer-mediated TiO2 MC formation.
O3 To evaluate TiO2 MCs formation in thin films.
O4 To generate mixed/hybrid mesocrystals (MMCs).

Year 1 of POLYCOMP centred on O1 and O2 - TiO2 MC formation in the presence of different polymers and surfactants. In addition modification of commercial polymers was attempted to further evaluate control over MC formation. Moreover the photocatalytic activity of these materials was also evaluated. POLYCOMP successfully demonstrated that the polymer, PEG-6000, can serve as an effective template for NH4TiOF3 MC formation Optimisation of this process resulted in anatase TiO2 MCs with good photocatalytic efficiencies, comparable with data reported elsewhere for such systems. The work also gave new insight on how Ti-PEG interactions serve to direct crystallisation and enabled a model to be developed for the MC formation process. Moreover, the photocatalytic activity observed for the TiO2 MCs indicates the PEG6000 promoted 001 facet formation. Since such control over morphology is critical for MC properties, eg photocatalysis this model will inform future MC syntheses. This work has now been published.

In O3 spin coating was used to make films containing TiO2 MCs with good, even MC distribution throughout the film, which could then be tested for photocatalytic activity. Results obtained here would indicate whether or not the film could potentially be used in certain applications eg self-cleaning windows. Many spin-coating processing parameters were optimised and films so generated were assessed for photocatalytic activity and shown to exhibit significantly greater photocatalytic activity than control films containing no TiO2 MCs.

In O4 initially work focused on generating TiO2 MC-quantum dot (QD) hybrid materials (MMCs) unfortunately, the QDs present in the MMC materials only gave very poor emissions and so at the start of the second year of POLYCOMP O4 was modified. As part of her training, the Scientist-in-Charge helped the Fellow successfully compete for and win time on the Diamond Light Source (DLS) synchrotron facility and the Fellow also established a new collaboration at the European Synchrotron Research Facility (ESRF). At the DLS and the ESRF, X-ray data was collected as single crystal and bulk samples of NH4TiOF3 MCs were heated. Subsequent data analysis has elucidated all the previously unknown structural changes that occur as NH4TiOF3 MCs are converted into TiO2 MCs. One paper on this work has been submitted, a further one is in draft.
POLYCOMP has made considerable progress beyond the state of the art. Most notably this is by generating entirely new insight into both the true crystalline structure of NH4TiOF3 and perhaps more even importantly in elucidating the key structural changes that accompany the complex series of 4 chemical processes that occur in the thermally-mediated tototactic transformation of NH4TiOF3 MCs into TiO2 MCs. With regards the true structure of NH4TiOF3, previously the only structural definition had been provided by Laptash who had suggested it as being isostructural with NH4FeF4 (based on a comparison of cell parameters). POLYCOMP results (SAXS and WAXS data specifically) suggest that in fact the true structure is not strictly isomorphous with NH4FeF4 and moreover demonstrates a reversible phase transition between a low (room temperature) and high (160oC) temperature phase with the latter high temperature phase enabling subsequent transformation ultimately into TiO2. Subsequent structural transformations to TiO2 have all been elucidated enabling for the first time the exact movements of the atoms within NH4TiOF3 to be mapped along with the chemical reactions so explicitly defining what occurs in the previously little understood transformation.

POLYCOMP has also enabled a working model on MC formation, specifically NH4TiOF3 MC formation to be developed. Again this shed light on a process that prior to POLYCOMP was not entirely understood. Specifically, determining the ratio of Ti atoms to templating polymer (poly(ethylene glycol) (PEG) enabled the POLYCOMP model to be developed and sheds light on how the Ti-PEG interactions serve to direct the crystallisation process. Although only empirical, this model allows for the effect of PEG concentration on the resultant MC morphology to be rationalized. In the case of the ideal PEG concentration, the number of nucleation sites fits nicely the volume of 3-dimensional matrix formed by the PEG. Nucleation then leads to oriented crystal growth which generates a crystallographically aligned cluster of nanocrystallites all initiated at roughly the same time and all growing at a similar rate until the precursors, within the polymer-bounded matrix are exhausted and perfect MCs result. If too little PEG-6000 is present there is an excess of inorganic MC precursor present relative to the number of polymer-based nucleation sites. This situation results in an additional process of crystallization, leading to the formation of a characteristic central defect region within the NH4TiOF3 MCs. Conversely excess PEG triggers reaction between similar units so agglomeration of individual MCs occurs resulting in clumping of the MCs into larger entities. These ideas have been published and fit with previous observations.