Theme D:

Aggregation-Based Mechanisms of Crystal Growth

Fiona Meldrum, James Elliott, Mike Allen, Roland Kröger

Helmut Cölfen, Nico Sommerdijk

Goals:• Aggregation-based crystal growth mechanisms• From crystalline to amorphous precursor particles• Mesocrystals• PILP

Nanoparticle interactions in solution (Elliott)

Studying the molecular interactions between calcite nanoparticles in aqueous solution, in order to deduce coarse-grained potentials of mean force for incorporation into larger scale models of aggregation (Warwick, year 4) and to reduce the calculation time required for more complex solvents using AdResS

PILP Phases of Calcium Carbonate (Meldrum/ Sommerdijk)

Investigated PILP formation in the presence of a series of random copoly(amino acid)s constructed from 80%-20%, 50%-50% and 20%-80% aspartic acid and serine residues

Infiltration of Collagen with PILP Phases (Meldrum/ Sommerdijk)

Studying the infiltration of collagen with calcium carbonate and iron oxides using SAXS/ WAXS and TEM. The collagen matrix provides remarkable control over the morphology and orientation of the occluded crystalline mineral phase in both the calcium carbonate and iron oxide systems, and that ultra-thin platelets are generated within the collagen.

High-Magnesian Calcite Mesocrystals (Sommerdijk/ Meldrum)

Liquid-Cell TEM Studies (Kroger)

A three port fluid cell holder has been purchased by Kroger to perform in-situ investigation of crystallisation processes under atmospheric conditions in a TEM. The cell holder has two input, and one output channel, which can be used to control the fluid composition.

High-Mg calcite crystals were formed in non-aqueous environments. Mg-containing ACC was produced by reacting a mixture of organometallic Ca and Mg complexes with CO2 and control over the Mg incorporation was obtained according to the ratio of the starting materials. Subsequent crystallization at reduced water activities in an organic solvent/ water mixture yields high-magnesian calcite mesocrystals.

Calcium Carbonate Mesocrystals: A Story

Fiona Meldrum, Yi-Yeoun Kim, Johannes Ihli, Anna

Schenk, Nicola Hetherington

What is a Mesocrystal?

“... a mesocrystal ideally comprises a 3D array of iso-oriented single crystal particles of size 1–1,000 nm. The highly oriented subunits therefore distinguish a mesocrystal from a randomly oriented polycrystal, and the identifiable nano-sized building units distinguish it from a single crystal containing impurities.”

Seto, Ma, Davis, Meldrum, Kim, Colfen et al PNAS 2012, 109(10), 3699.

Evidence for a Mesocrystal ?

1. Morphology – evidence for nanoscale subunits (SEM analysis)

2. XRD – line broadening used as evidence of sub-structure

3. Ultrastructure - evidence for nanoscale subunits (TEM analysis)

4. Surface Area – large surface areas used as evidence of sub-structure (porosimetry)

Poly(styrene sulfonate) (PSS)

“Classic” Synthetic CaCO3 Mesocrystals

Poly(styrene-alt-maleic acid) (PS-MA)

Poly(styrene sulfonate – co – maleic acid) (PSS-MA)

Review Evidence for Mesocrystal Structure - SEM

Poly(styrene-alt-maleic acid) (PS-MA) Cobalt ions

• Observe similar morphologies / surface roughness

• Sample precipitated with Co is a single crystal well-studied in the literature

Seeded Growth

Characteristic morphologies can be produced simply by growing a rhombohedral calcite seed crystal in the presence of the polymer additives

2 -3 m seed

Morphology is not defined at nucleation/ early stages of growth

XRD Analysis – Line Broadening

Commonly seen in literature line broadening attributed to small particles:

Scherrer equation:

However...

LATTICE STRAIN ALSO CAUSES BROADENING OF XRD PEAKS!!

• Relationship of broadening with diffraction angle () different

• From analysis of whole spectrum one can derive the separate contributions of particle size and strain to the line broadening

t = crystallite size = FWHM K = shape factor

High Resolution Synchrotron Powder Analysis of Calcite Mesocrystals

2 independent analyses (by people who don’t

care...)

• Line broadening due entirely to LATTICE STRAIN !

• No nanoparticulate sub-structure

2 separate batches

PSS-MA 800 nm domain PS-MA 1000 nm domain

• Distinct interface along the edge, but continuity of crystal orientation and single crystal structure

• No evidence for nanoparticulate substructure

Ultrastructure – TEM Analysis (FIB)

Surface Area - Porosimetry

Challenging and Interesting !

• Helmut’s calcite mesocrystals surface areas > 100 m2/g

• Repeated experiments values < 10 m2/g

• What is going on ??

Surface area is time-dependent !

• If you analyse crystals freshly removed from the solution (after any ageing time), surface areas > 100 m2/g

• HOWEVER, if you leave a sample in air for 5-7 days before analysing the surface area, surface area < 10 m2/g

• It has been suggested that mesocrystals undergo a recrystallization with time

• Do the internal nanoparticles fuse, causing a reduction in surface area ?

Synchrotron XRD analysis :

• See no change in the line broadening between new and old samples

• Further, see no change in line width after heating a sample to 400 oC

Results suggest that the difference in surface areas on aging is a SURFACE EFFECT possible blocking of surface pores ??

Evidence for Calcite Mesocrystals - Mechanism of Formation?

Wang, Colfen, Antonietti, JACS, 2005, 127, 3246.

• ACC particles formed

• ACC particles rapidly aggregate

• Subsequent crystallization

No evidence for crystalline nanoparticles has been

obtained (to-date)

Characterisation of the Ammonia Diffusion Method

Common observation – “mesocrystal” morphologies only formed with diffusion-based syntheses

• Nucleation event consumes only a minor fraction of the Ca ions

• Superaturation remains constant and well above the solubility level of ACC for the vast majority of the reaction

New material is constantly generated throughout the reaction using the ADM

A Possible Scenario?

• CaCO3 mesocrystals based on the crystallization of assemblies of ACC nanoparticles rather than the oriented assembly of precursor crystalline nanoparticles

• New particles nucleate on existing polymer-stabilized ACC aggregates, or on crystalline particles at later stages of the reaction, giving rise to more complex morphologies.

• Ostwald ripening can be important in generating key mesocrystal morphologies. Even the slowest reaction conditions, the ADM is complete and the calcium ions depleted after 6-8 hours.

• Morphological changes in crystals after 12 hrs (days to weeks have been described), are due to Ostwald ripening/ recrystallization in the presence of polymer.

• Synthetic CaCO3 mesocrystals may resemble biogenic calcite mesocrystals, where the ultrastructure derives from a memory of the ACC precursor phase.

Biogenic Mesocrystals – Sea Urchin Spines

“.... the term mesocrystal defines the structure of a material rather than its mechanism of formation.”

“ ... while oriented aggregation of crystalline nanoparticles can give rise to either a single crystal or mesocrystal product ... a mesocrystal can also form when a dense array of amorphous nanoparticles crystallizes to give a highly co-oriented end-product material.”

Seto, Ma, Davis, Meldrum, Kim, Colfen et al PNAS 2012, 109(10), 3699.

NMR, SAXS • Residual ACC • Consistent with nanoparticles coated with ACC

Summary

• Evidence for calcite “mesocrystals” poor no evidence for oriented assembly

• Complicated by dominance of ACC precursor phase

• Residual mesoscale structure may originate as a memory of ACC precursor phase

Modelling:

• Steer well clear of calcite as a model “mesocrystal”

• Address a well-characterised system (eg iron oxides)

Experiment:

• Development of morphologies, crystal growth mechanisms interesting

• Mechanism of crystallization of ACC phase (link with modelling)

Oriented AttachmentFerrihydrite Nanoparticles Transform to Goethite Single

Crystals

Yuwono, Burrows, Soltis, R. Lee Penn J. Am. Chem. Soc., 2010, 132 (7), 2163.

[Ca] = 10 mM [Ca] = 5 mM [Ca] = 2.5 mM

[Ca] = 1 mM [Ca] = 0.5 mM [Ca] = 0.1 mM

Kulak AN .... Meldrum FC (2007) J. Am. Chem. Soc. 129, 3729-3736.

Ultrastructural Analysis of “Meoscrystals”

Investigating particles between polycrystals and mesocrystals, cutting using FIB, TEM analysis

Investigation of Mechanism

A

1µm

B

DC

1µm

1µm

1µm

ADM Titration method

Titration-based method:

• Highly reproducible

• Can study the reaction with time using DLS, TEM

• We will do initial dry TEM studies, Nico will do cryo-TEM of key samples

• Longer term in situ TEM with fluid cell

• Challenges associated with getting these key morphologies in that experimental set-up

Electron Backscatter Diffraction

Erika Griesshaber, Wolfgang Schmahl, Munich

Schematic model for shell growth with a microstructure consisting of splined and interdigitating grains. ACC is present in confined capsules delineated by vesicle membranes

Goetz, Steinmetz, Griesshaber, Schmahl et al Acta Biomaterialia, (2011), 7, 2237.

Brachiopod