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)

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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

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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.)

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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!