Dense Intermediate liquid and amorphous phases as an Alternative Model for Oxide Nanoparticle Synthesis – DIAMONS

The synthesis of crystalline oxide nanoparticles by co-precipitation of ions in water and at room temperature is an industrially appealing method to produce functional nanomaterials: it is suitable for up-scaling; it shows reduced environmental and energetic impact; its generic character allows transposition to virtually any transition metal oxide nanoparticles, offering a broad catalogue of properties for applications. Yet, the development of functional materials from nanoparticles requires an optimal control over their crystallinity (quality and polymorph selection) and nanostructure (size, aggregation state). In the case of crystalline oxide nanoparticles synthesized at room temperature in water, a crucial bottleneck which hampers this control is the failure of the classical models to describe the multi-step process of nucleation at work in such “soft chemistry” syntheses.
The new insight brought by the DIAMONS project is to investigate the successive transient states from the solution to the nanocrystals, to understand how they template the crystalline oxide nanoparticles, and how they select the nanoparticle structure/polymorph (Figure 1). These transient states are (i) dense liquid phases that form even before any chemical reaction has occurred, or (ii) a possible succession of amorphous states that develop prior to crystallisation. As both kinds of transient states are presumably ubiquitous in syntheses of oxide nanoparticles in water, their identification is critical in order to reach an optimal control of nanostructure and crystalline lattice, beyond the rule-of-thumb provided by the classical nucleation theory. The implications of the multi-step nucleation are to date poorly explored, not only because of the novelty of the concept, but also because of the great experimental challenge to overcome: the characterisation of transient states from the Angstrom to the submicron scale, at reaction times shorter than one millisecond.
We will face the challenge using as model cases the synthesis in water of luminescent rare earth-doped vanadates nanoparticles, namely YVO4:Eu and LaVO4:Eu. In these systems, the control of nanostructure and/or polymorph drastically condition their emission properties, hence their potential as luminescent biolabels or precursor for thin-film phosphors. We will monitor the transient states for reaction times starting from about hundred microseconds using novel microfluidic developments will be coupled with (i) standard laboratory/synchrotron-based X-ray structural characterisations (SAXS, WAXS, EXAFS), (ii) time-resolved luminescence spectroscopy to probe local dynamics and chemical environment, and (iii) total scattering measurements with pair distribution function analysis (PDF) which provides a full description of the local order in disordered media. These experimental observations will constitute input for theoretical modelling based on cutting-edge developments in phase field theory and statistical physics. Such combination of theory and time-resolved, in situ structural and dynamic characterisations will provide generic rules for the control of the morphology and polymorph selection of oxide nanoparticles relevant for their production at industrial level.