Crystallisation of nano-quartz and nano-graphite from microemulsions under ambient conditions.

Abstract

The principal basis for the following work is to demonstrate the importance of controllable crystallisation and consequential applications for both silica-based significant inorganic materials and carbonaceous materials. Microemulsions have been employed as a vector to explore the possibility of thermodynamically controlling the crystallisation process, utilising the 3D confinement of crystallisable material within nano-scale droplets. This opens a route to circumvent Ostwald’s Rule of Stages, for a multitude of potential applications.
We show here that both quartz and nanographite can be synthesised at room temperature and pressure using this methodology.
Previous attempts at silica synthesis from within the microemulsion have only presented the amorphous phase, leaving many questions unanswered, whilst failing to reveal the underlying cause. Further, traditional methods of quartz synthesis employs hydrothermal conditions, or temperatures >1100 0C. Microemulsions were adopted to behave as confined mini reactors for the synthesis of α-quartz at room temperature and pressure from a precursor from sodium metasilicate nonahydrate (SMS) which can be used as a precursor of silica, circumnavigating the traditional hydrothermal methodologies. At higher supersaturations, both the metastable amorphous phase and the high temperature polymorph, cristobalite were also observed. Upon the acidification of the microemulsions, the size and morphology of the quartz nanoparticles was found to be dependent upon the pH and the ratio of surfactant:silica units.
Conventional wisdom stipulates that graphite can only be produced using high temperatures, with natural graphite arising via progressive metamorphisms of carbonaceous material subjected to temperatures above ~600 K and pressures >2 kbar. Previous attempts to use carbohydrate precursors have resulted in the formation of luminescent carbon dots or required templation, followed by calcination. In these prior investigations, high temperatures or extremely severe reactants are used to drive the precipitation of graphitic forms.

Analogous experiments were successful in employing the 3D nano-confinement microemulsions as confined mini reactors for the synthesis of nanographite at room temperature and pressure from a sucrose precursor, through a simple process of acidifying sucrose microemulsions.
Crucially, the reaction was conducted in nanometre-sized microemulsion droplets to exert control over the reaction and sheet stacking process, ensuring that only sufficiently pristine graphene nanosheets could stack, thereby producing nanographite in a simple one-step synthesis under ambient conditions. The primary nanographitic particles of size ~3-30 nm stacked to form larger µm-sized nanographitic aggregates. The amount of nanographite produced from the microemulsions is limited as sucrose concentration must be kept very low to slow the reaction kinetics to ensure the mainly graphitic, rather than amorphous, product.