Understanding confinement effects on crystallisation using an imaging approach

Fiona Meldrum and her team are looking at what happens to crystals growing in nano-sized spaces

Does your kettle fur up with limescale? As a child, were you encouraged to drink cow’s milk to build strong bones? If a cyclist, have you ever cursed the cracks in the road that appear after ice and snow? Each constitutes an everyday encounter with crystallisation. It lies at the heart of a vast number of processes as varied and important as the synthesis of pharmaceuticals, ceramics and nanomaterials. Crystallisation is responsible for the formation of scale in heating systems and oil wells as well as the humble kitchen kettle. It also underlies environmental processes such as weathering and frost-heave (the cracks in the road), and the generation of biominerals such as seashells, bones and teeth.  Understanding the mechanisms which govern crystallisation therefore promises the ability to inhibit or promote crystallisation as desired, and to tailor the properties of crystalline materials towards a huge range of applications.

In both synthetic and natural systems crystals often grow differently within a confined volume of solution – and the development of minutely structured nanomaterials means that we can now grow crystals in tiny tubes just nanometres in diameter. Perhaps the most intriguing effect is on the structures of the crystals produced.  Calcium carbonate is one of several substances that forms crystals in which its constituent ions are in different arrangements.  Such crystals are termed ‘polymorphs’.  This is important as different polymorphs have different properties, such as solubility.

Our research seeks to understand the effects of confinement on crystallisation by working out how individual crystals evolve in defined micro-environments.  We will use advanced imaging techniques to determine when and where the first crystals appear within a defined volume; and how the number and distribution of crystals vary with time, and the polymorphs, size and shape of the crystals develop.  

Transmission electron microscope images of (a) empty titania Ti02 nanotubes and (b) amorphous calcium carbonate precipitates
within the nanotubes (shown by arrow) before the crystals take shape

We will use various systems, each offering different shapes (eg wedge, cylindrical pore) and which we can study using different microscopy methods.  For example, we will use electron microscopy to study crystals growing in titania nanotubes made up of isolated cylindrical pores of diameter 10–200 nm (pictured). X-rays will give us a 3D image of crystals grown in controlled pore glasses’ sponge-like network of pores just 5-300 nm in diameter.  Graphene oxide ‘pockets’ have quite different shapes, and provide a unique chance to image the way the crystal grows from the smallest particle to the final product.  We will also use a new type of electron microscopy which makes it possible to look at the crystals growing in solution in real time at very high magnifications, to study how the crystals form and then grow in small volumes.  These experiments will be supported by modelling studies to enhance our understanding of confinement effects on crystallisation. 

Together, these investigations will enable us to answer the question ‘how does confinement affect the ways that crystals grow?’

*a micron, μm =   0.001 mm; a nanometre, nm = 0.000001 mm

Professor Fiona Meldrum
University of Leeds
Research Project Grant