LA FISICA DEI MINERALI: IMPLICAZIONI GEOLOGICHE E APPLICAZIONI PRATICHE
2-5 Febbraio 2015 - Bressanone
Physical properties and chemical reactivity at the nanoscale: the role of surfaces
Gilberto Artioli UNIPD – CIRCe Centre
We are addressing rather fundamental questions:
What is a nano-mineral? (i.e. atom/molecules vs nuclei vs crystals)
Properties of nanomaterials vs bulk (i.e. the state and role of surfaces)
Dimensionality
3D nanoparticles
2D nanoparticles (graphene, single-layer silicates)
1D nanoparticles (fibres, asbestos, nanowires, nanotubes)
Nanominerals and mineral nanoparticles, whatever their origin, seem to be ubiquitous, and for the
most part they went unnoticed until recently. They were, in more ways than one, the ‘invisible’ part
of the mineral kingdom. They were also completely misconstrued in the field of aqueous
geochemistry when, for decades, anything that went through a 0.45-micron filter was considered
‘dissolved’. (nanominerals are coming alive !!!)
The most fundamental part of this field, summarized in just one question in the list of 100, is how
the chemical and mechanical properties of minerals vary as a function of their crystal size and
shape in the nano-range of sizes (from one to a few tens of nanometers). This notion of property
variation as a function of size provides the foundation of every subdiscipline under the now enormous
and important field of nanoscience. (nanoscience: let’s discover the properties of nanoparticles!)
Our survey shows that nanomineralogists are asking questions about the role that nanominerals
and mineral nanoparticles play in (bio)geochemical processes at the local, regional and global
scales; about how they affect life on Earth; and about the complex inventory and reactivity of perhaps
the largest accumulation of nanominerals and mineral nanoparticles on the near-surface of the
planet, that is, in the world’s oceans. One question addresses perhaps the latest frontier in this rapidly
growing field, namely the effort to understand the amount, distribution, and reactivity of poorly
crystalline, inorganic nanomaterials in soils, sediments and surface waters.
(what’s the role of nanominerals in the geo-processes?)
Crystals → Long range order (LRO)
Smaller Crystals Produce Broader XRD Peaks
βε = Cε tanθ Mean strain effect on peaks:
We can use the Williamson-Hall plot (Bcosθ) vs (sinθ) To extract the information on size and strain
Diffraction profiles from (micro)crystalline materials
• Materials with (sub)micrometric scattering domains (ideal crystals)
• The intensity in reciprocal space is concentrated in the Bragg positions
• The peak position is determined by the lattice dimensions
• The integrated intensity is determined by the structure factor (unit cell chemical content)
• The pattern resolution is dominated by the instrumental broadening
LaB6
SLS-MS synchrotron – high resolution
(13 KeV, λ = 0.95 Å)
Hydroxyapatite
Laboratory data (8 KeV, λ = 1.54 Å)
10
Au NPs - D∽4nm
LNSL (8.040 KeV, λ =1.542 Å)
Au NPs - D∽2nm
LNSL (8.040 KeV, λ =1.542 Å)
Diffraction profiles from nanocrystalline materials
• Materials with nanometric scattering domains (non-ideal crystals)
• The intensity in reciprocal space is diffused between the Bragg peaks
• The peak position are shifted with respect to the lattice dimensions
• The pattern resolution is dominated by the sample broadening
12
Very often, when we deal with real materials, we do have a number of samples of mixed or intermediate nature.
Phases with stacking disorder (i.e. clays, oxides, etc.), defects, poor periodicity (xerogels, polymers, etc.)
14
Atoms and molecules → Short range order (SRO)
radial distribution function
g(r)
Fig. 1 Molecular structures and laboratory Cu Ka1 XRPD patterns for X-ray
amorphous melt-quenched samples of CBZ (top) and IND (bottom).
Fig. 2 Total scattering diffraction patterns and TSPDFs of CBZ. Panels (a) and
(d) correspond to CBZ III, (b) and (e) to the melt-quenched sample and (c) and
(f) to CBZ I; (a), (b), (c) show the total scattering data in the form of F(Q) (see
ESI)† whilst (d), (e), (f) are in the form of the TSPDF, G(r).
Fig. 3 Comparison of TSPDF from the melt-quenched amorphous sample
(green) and CBZ III (blue), modified as if it were a 4.5 nm nanoparticle (see
text for details). PolySNAP correlation coefficient 0.8601.
Neutron pair distribution function
analysis of nano (black, top) and
bulk (light blue) BaTiO3.
Benzyl alcohol scattering is dark
blue.
Bulk BaTiO3 is described nicely
by P4mm structure, but to fit the
nanoparticle G(r), contributions
from both P4mm BaTiO3 and the
benzyl alcohol are needed.
size
Order / periodicity
SRO
LRO
MRO / mesocrystals
Mesocrystals: how to play with nucleation and growth !
Four principal possibilities to explain the
3D mutual alignment of nanoparticles to
a mesocrystal.
a) Alignment of nanoparticles by an
oriented organic matrix.
b) Nanoparticle alignment by physical
fields or mutual alignment of
identical crystal faces. The arrows
indicate the mutual alignment by
physical fields or the faces.
c) Epitaxial growth of a nanoparticle
employing a mineral bridge
connecting the two nanoparticles.
d) Nanoparticle alignment by spatial
constraints. Upon growth of
anisotropic nanoparticles in a
constrained environment, the
particles will align throughout
growth according to the space
restrictions as indicated by the
open drawn particle in the
arrangement of already grown
particles in the lower image of (d).
Please note that the building units are
shown to be monodisperse for the sake
of clarity. This is not always the case in
real systems. Also, the mutual order is
not necessarily that of the shown
crystallographic register.
CrystEngComm, 2011, 13, 1249
Templated single crystal of calcite
precipitated in a sponge like polymer
membrane from 0.02 M reagents A calcite single crystal with gyroid
morphology after removal of the PS template
Hydroxyapatite whiskers grown in an
aggregate of hydrophobically modified
polyethylenoxide-block-polymethacrylic
acid block copolymer
Properties = f (order, structure, size)
TEM micrograph of silver nanoparticles synthesized by Aspergillus fumigatus Read more: Fungus as bionanofactory for synthesis of silver nanoparticles
Two-dimensional models of solid state (right side) and corresponding band models (left side): (a) crystal, (b) amorphous semiconductor, (c) amorphous insulator. The region of localized states are shaded.
There are basically two types of size-dependent
effects:
smoothly scalable ones which are related to the
fraction of atoms at the surface
quantum effects which show discontinuous
behaviour due to completion of shells in systems
with delocalised electrons
Scalable physical propertes
Figure 1. Fraction of the volume of a spherical particle within 0.5 nm of its
surface as a function of its diameter. The lighter colored shell surrounding
the dark core represents this fraction; it can also be viewed as the volume
fraction of a 0.5 nm coating the system can carry.
Evolution of the dispersion F as a function of n for cubic clusters up
to n = 100 (N = 106).
Calculated mean coordination number <NN> as a function of inverse radius,
represented by N-1/3, for magnesium clusters of different symmetries (triangles:
icosahedra, squares: decahedra, diamonds: hexagonal close packing).
Au bulk Nano <10 nm
colour shiny yellow red
structure fcc cubic Icosahedral -
planar
magnetic char Non magnetic magnetic
electric conduct metallic insulator
Melting T 1336 K << 1336 K
activity noble excellent
catalysts
A well-known example of catalytic activity of a nanomaterial is gold. In the bulk
form gold is inert; however, gold nanoparticles of several nm in diameter
acquire spectacular catalytic properties. For example, at low temperatures
nanosized gold is the best catalyst for CO oxidation.
Background-subtracted differential scanning calorimetry melting endotherms for indium confined in
controlled pore glass (a–c) and in Vycor samples (d) with different pore diameters. Note that the melting
feature of the pore-confined material moves to lower temperatures and broadens as the pores get narrower.
Right: Melting temperature as a function of pore diameter and inverse diameter. The broken line represents
the bulk melting point
Gibbs–Thomson equation following from
consideration of the relative thermodynamic
contributions of surface and bulk energies
R radius of spherical nano-particles
H latent heat of fusion
mass density
interface energy
<<1 is the liquid-layer thickness
normalized by R (Peters et al. 1998)
Homogeneous melting and growth model Liquid shell nucleation model Liquid nucleation and growth model
Wang et al. – Proc. Roy. Soc. A, 462, 1355-1363, 2006
For homogeneous nano-structured materials:
the surface elastic constant
E Young modulus of the bulk material
Ab initio and molecular dynamic simulations, and experimental results show
that the elastic constants of nano-plates, nano-beams and nano-wires obey
the scaling law above almost exactly in and above the range 1–100 nm
LiIO3
ground powder
as synthesised
It is striking that, in general, as the polymorph becomes more metastable as a
coarse phase, its surface energy diminishes. This competition between the
energetics of polymorphism and surface energy leads, in general, to
crossovers in the stability of polymorphs at the nanoscale.
(Navrotsky, ChemPhysChem 12, 2207-2215, 2011)
Quantum effects
Pointillism
Georges-Pierre Seurat (1859-1891)
'post-pointillist' painting
A dot painting made of photonic crystals can make colour without normal dyes or pigments
Fluorescing CdSe-CdS nanoparticles. The smaller the particle, the larger the band gap, and, consequently, the shorter the emitted wavelength [diameter of 1.7 nm (blue) to 6 nm (red)].
Fig. 1 Study of absorption and scattering plasmonic optical properties of colloidal Ag NPs using UV-vis absorption spectroscopy.
Channel-type nanostructures consist of
beta-keratin bars and air channels in elongate
and tortuous forms. Sphere-type
nanostructures consist of spherical air
cavities in a beta-keratin matrix.
Recent studies suggest these nanostructures
are self-assembled during phase separation
of beta-keratin protein from the cytoplasm of
the cell. The channel morphology is
developed via spinodal decomposition and
the sphere morphology, via nucleation and
growth.
Photonic crystals is a new class of
materials whose properties are
determined by the dielectric
constant periodicity in real space
which, in turn, induces a periodic
energy spectrum with bands and,
eventually, gaps in reciprocal space.
Our approach to the problem is the
synthesis of artificial opals.
Because of the quantised states of electrons and holes these nanocrystallites
are often called quantum dots, pseudo-atoms or superatoms. The core–shell
structure serves to control the potential that confines the electrons and
determines the lifetime of excited states.
Fig. 8 Ionisation potentials and electron
affinities of elements (upper tableau) in
comparison with measured (circles)
and on the basis of a shell model
calculated (line) vertical electron
affinities of Au1-70 as a function of
cluster size (lower tableau). Even
numbered cluster sizes are marked
with a full dot.
Examples of delocalised systems with high symmetry: C60 (fullerene) and Au32 (gold). Both are cage-like hollow clusters.
Magnetization at 1.8 K of the Pt13NaY before (solid dots, lower curve) and after
hydrogen desorption (solid dots, upper curve), compared with literature data for
analogous measurements of Pt nanoparticles embedded in a PVP polymer. The
particles have diameters of 2.3 nm (ca. 420 atoms), 3.0 nm (940 atoms) and 3.8
nm (1900 atoms).
Absolute rate constants for the reaction with N2O of cationic (squares)
and anionic (open circles) Pt(n) clusters. Some of the lowest values
represent upper limits of the rate constant for unreactive cluster sizes.
engineered
(functionalized)
nanoparticles
nm scale
m scale
> mm scale
Hydration process
The seeds can be produced with different kind of structural order, with respect to the tobermoritic structural model of CSH
Hydration process
Nano-minerals in nature: (A) 2-D nanosheets (vernadite); (B) 1-D nano-rods (palygorskite); (C) 3-D Nanoparticles (ferrihydrite) (Elements, 2008, 4, p. 376)
Nanocrystalline minerals are ubiquitous in natural systems.
They are characterized for having coherent domain sizes in
the nanometer range, high specific surface areas and,
usually, colloidal properties.
All these properties make them important environmental
sinks of pollutants and contaminants, as well and vectors for
the colloidal transport of contaminants in the environment.
The high density of broken bonds at their surfaces often
allows for exceptional catalytic activity, and their frequent
imperfect stoichiometry, that results from low-temperature
and (or) of biogenic crystallization often leads to the presence
of mixed-valent structures that possess a redox potential
allowing for the degradation of molecules such as organics.
On the other hand, mineral nanoparticles «the ‘nano’
version of bulk minerals» can form as the result of
weathering or dissolution processes, under conditions
of limited mineral growth, or even as transient phases
during biotic and abiotic mineral formation processes.
The advent of advanced characterization techniques for the
detection of nanominerals and mineral nanoparticles in
natural systems, as well as for their structural study has
extended the now well established nanotechnology
approaches to the mineralogical science.
Nanogeoscience
• Are there nano-effects present in nature and how do they influence the cycles of matter on
Earth?
• What is the role of natural nanoparticles in solute transport in Earth systems?
• What is the relation between the genesis and properties of mineral nanostructures?
• How and why do the chemical and mechanical properties of minerals vary as a function of
crystal size and shape, something that is expected to occur as grain size is reduced into
the nano-scale?
• How do nanominerals and mineral nanoparticles influence macroscopic (bio)geochemical
processes at local, regional, and global scales?
• How do nanominerals and mineral nanoparticles, as well as manufactured nanoparticles
inadvertently entering the environment, influence/affect life on Earth?
• What is the inventory of mineral nanoparticles in the world's oceans, and what
biogeochemical role do they play, including the role they play in supplying limiting nutrients
to the microorganisms of the oceans?
High surface → extreme reactivity → toxicity !?
Industrial preparation → stability vs reactivity
Thank you
for your attention !
Nanomineralogists need plenty of different tools!
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