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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 Attachment
Ferrihydrite 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