Transcript CHM 434F/1206F SOLID STATE MATERIALS CHEMISTRY

CHARGE BALANCE IN SOLID STATE INTERFACIAL
REACTIONS
• 3Mg(2+) diffuse in opposite way to 2Al(3+)
• MgO/MgAl2O4 Interface
• 2Al(3+) - 3Mg(2+) + 4MgO  MgAl2O4
LHS
• MgAl2O4/Al2O3 Interface
• 3Mg(2+) - 2Al(3+) + 4Al2O3  3 MgAl2O4
RHS
• Overall Reaction
• 4MgO + 4Al2O3 4 MgAl2O4
• RHS/LHS growth rate of interface = 3/1 Kirkendall
Effect
KIRKENDALL EFFECT
OTHER SOLID STATE REACTIONS
• MgO + Fe2O3 MgFe2O4
• Different color interfaces
• Easily monitored rates
• Other examples - calculate the Kirkendall ratio:
• SrO + TiO2  SrTiO3 Perovskite, AMO3 (type ReO3)
• 2KF + NiF2 K2NiF4 Corner Sharing Oh NiF6(2-)
Sheets, Inter-sheet K(+)
• 2SiO2 + Li2O Li2Si2O5
ROCK SALT CRYSTAL STRUCTURE
M
O
y
x
RUTILE CRYSTAL STRUCTURE
z
y
x
PEROVSKITE CRYSTAL STRUCTURE
M
O
A
PEROVSKITE MATERIALS: STRUCTURE-PROPERTYFUNCTION-UTILITY RELATIONS
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LiNbO3 non-linear optical ferroelectric - E-field RI control - electrooptical
switch
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SrTiO3 dye sensitized semiconductor liquid junction photocathode - solar
cell
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HxWO3 proton conductor - hydrogen/oxygen fuel cell electrolyte
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BaY2Cu3O7 high Tc superconductor - magnetic levitation - detector/SQUIDS
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BaTiO3 ferroelectric high dielectric capacitor, photorefractive – holography
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CaxLa1-xMnO3 x control F-metal, P-semiconductor - GMR - data storage
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SrxLa1-xMnO3 x control e-/oxide ion conductor - solid oxide fuel cell cathode
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PbZrxTi1-xO3 piezoelectric - oscillator, nano-positioner
ASPECTS OF SOLID-SOLID REACTIONS
• Conventional solid state synthesis - heating
mixtures of two or more solids to form a solid
phase product.
• Unlike gas phase and solution reactions
• Limiting factor in solid-solid reactions usually
diffusion.
• Fick’s law : J = -D(dc/dx)
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J = Flux of diffusing species (#/cm2s)
D = Diffusion coefficient (cm2/s)
(dc/dx) = Concentration Gradient (#/cm4)
ASPECTS OF SOLID-SOLID REACTIONS
• The average distance a diffusing species will travel
<x>
• <x>  (2Dt)1/2
where t is the time.
• To obtain good rates of reaction you typically need
the diffusion coefficient D to be larger than ~ 10-12
cm2/s.
• D = Doexp(Ea/RT) diffusion coefficient increases
with temperature, rapidly as you approach the
melting point.
• This concept leads to Tamman’s Rule : Extensive
reaction will not occur until temperature reaches at
least 1/3 of the melting point of one or more of the
RATES OF REACTIONS IN SOLID STATE SYNTHESIS
ARE CONTROLLED BY THREE MAIN FACTORS
1. Contact area: surface area of reacting solids
2. Rates of diffusion of ions through various
phases, reactants and products
3. Rate of nucleation of product phase
Let us examine each of the above in turn
SURFACE AREA OF PRECURSORS
• Seems trivial - vital consideration in solid state
synthesis
• Consider MgO, 1cm3 cubes, density 3.5 gcm-3
• 1 cm cubes: SA 6x10-4 m2/g
• 10-3 cm cubes: SA 6x10-1 m2/g
• 10-6 cm cubes: SA 6x102 m2/g
• The latter is equal to a 100 metre running track!!!
• Clearly reaction rate influenced by SA of
precursors as contact area depends roughly on SA
of the particles
EXTRA CONSIDERATIONS IN SOLID STATE
SYNTHESIS
• High pressure squeezing of reactive powders into
pellets, for instance using 105 psi
• Pressed pellets still 20-40% porous
• Hot pressing improves densification
• Note: contact area not in planar layer lattice diffusion
model for thickness change with time, dx/dt = k/x
EXTRA CONSIDERATIONS IN SOLID STATE
SYNTHESIS
• x  1/A(contact)
• A(contact)  1/d(particle)
• Thus particle sizes and surface area connected
• Hence x  d therefore A and d affect interfacial
thickness
• These relations suggest some strategies for rate
enhancement in direct reactions
MINIMIZING DIFFUSION LENGTHS <x>  (2Dt)1/2 FOR
RAPID AND COMPLETE DIRECT REACTION BETWEEN
SOLID STATE MATERIALS AT LOWEST T
Particle surface area A
Product interface thickness x
Particle size d
dx/dt = k/x = k’A =k"/d
All aimed to increase A and decrease x
and minimize diffusion length scale
Decreasing particle size to
nanocrystalline size
Hot pressing densification of particles
Atomic mixing in composite
precursor compounds
Coated particle mixed component
reagents, corona/core precursors
MINIMIZING DIFFUSION LENGTHS <x>  (2Dt)1/2 FOR
RAPID AND COMPLETE DIRECT REACTION BETWEEN
SOLID STATE MATERIALS AT LOWEST T
Core-corona reactants in intimate
contact, made by salt precipitation, solgel deposition, CVD
All aimed to increase A and decrease x
and minimize diffusion length scale
COATED PARTICLE
SOLID STATE REAGENTS
MINIMIZING DIFFUSION LENGTHS <x>  (2Dt)1/2 FOR
RAPID AND COMPLETE DIRECT REACTION BETWEEN
SOLID STATE MATERIALS AT LOWEST T
SUPERLATTICE
REAGENTS
•Johnson superlattice precursor
•Deposition of thin film reactants
•Controlled thickness, composition
•Metals, semiconductors, oxides
•Binary, ternary compounds
•Modulated structures
•Solid solutions
•Diffusion length control
•Thickness control of reaction rate
•Low T solid state reaction
•Designer element precursor layers
•Coherent directed product nucleation
•Oriented product crystal growth
•LT metastable heterostructures
•HT thermodynamic product
ELEMENTALLY
MODULATED
SUPERLATTICES DEPOSITED AND
THERMALLY POST
TREATED
COMPUTER
MODELLING OF
SOLID STATE
REACTION OF
JOHNSON
SUPERLATTICE
ELEMENTALLY MODULATED
SUPERLATTICES
• Give several important synthetic parameters and in
situ probes.
• Reactants prepared using thin film deposition
techniques and consist of nm scale layers of the
elements to be reacted.
• One element can be easily substituted for another,
allowing rapid surveys over a class of related
reactions and synthesis of isostructural
compounds.
ELEMENTALLY MODULATED
SUPERLATTICES
• The diffusion distance is determined by the
multilayer repeat distance which can be
continuously varied.
• An important advantage, allowing experimental
demonstration of changes in reaction mechanism
as a function of inter-diffusion distance.
• Multi-layer repeat distances can be easily verified in
the prepared reactants using low angle X-ray
diffraction.
MINIMIZING DIFFUSION LENGTHS <x>  (2Dt)1/2 FOR
RAPID AND COMPLETE DIRECT REACTION BETWEEN
SOLID STATE MATERIALS AT LOWEST T
Johnson superlattice reagent design
{(Ti2Se)6(Nb2Se)6}n
Low T annealing reaction
{(TiSe2)6(NbSe2)6}n
SUPERLATTICE REAGENTS
YIELD SUPERLATTICE
ARTIFICIAL CRYSTAL
PRODUCT
Metastable ternary modulated layered
metal dichalcogenide (hcp Se2- layers,
Ti4+/Nb4+ Oh/D3h interlayer sites)
superlattice well defined PXRD - PTO
Confirms correlation between
precursor heterostructure sequence and
superlattice ordering of final product
Superlattice sequence 6(Ti2Se)-6(Nb2Se) yields ternary
modulated superlattice composition {(TiSe 2)6(NbSe 2)6}n
with 62 well defined PXRD reflections
Confirms correlation between precursor heterostructure
sequence and superlattice ordering of final product
MINIMIZING DIFFUSION LENGTHS <x>  (2Dt)1/2 FOR
RAPID AND COMPLETE DIRECT REACTION BETWEEN
SOLID STATE MATERIALS AT LOWEST T
John superlattice reagent design
{(Ti2Se)6(Nb2Se)6}n
High T annealing reaction
{(Ti0.5Nb0.5Se2)}n
Thermodynamic Vegard type solid
solution ternary metal dichalcogenide
product with identical layers
SUPERLATTICE REAGENTS
YIELD HOMOGENEOUS
SOLID SOLUTION PRODUCT
Properties of ternary product is the
atomic fraction weighted average of
binary end member components
NANOCLUSTER KIRKENDALL SYNTHESIS OF
HOLLOW NANOCLUSTERS
S
Co
V[Co]
e(-)
Co(2+)
S(2-)
Co2S3
Capped cobalt nanoclusters in a high temperature solvent, sulfur injection, coating of sulfur on
nanocluster, cobalt sesquisulfide product shell layer formed at interface, counter-diffusion of Co(2+)/2e(-)
and S(2-) across thickening shell, faster diffusion of Co(2+) creates V[Co] in core, vacancies agglomerate
in core, hollow core created which grows as the product shell thickens – end result – a hollow nanosphere
made of cobalt sesquisulfide Co2S3
TURNING NANOSTRUCTURES INSIDE-OUT
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The Kirkendall effect is a well-known phenomenon discovered in the 1930’s.
It occurs during the reaction of two solid-state materials and involves the
diffusion of reactant species, like ions, across the product interface usually
at different rates.
In the special case when the movement of the fast-diffusing component
cannot be balanced by the movement of the slow component the net mass
flow is accompanied by a net flow of atomic vacancies in the opposite
direction.
This effect leads to Kirkendall porosity, formed through supersaturation of
vacancies into hollow pores
When starting with perfect building blocks such as cobalt nanocrystals a
reaction meeting the Kirkendall criteria can lead to supersaturation of
vacancies exclusively in the center of the nanocrystal.
This provides a general route to hollow nanocrystals out of almost any
given material
Proof-of-concept - synthesis of a cobalt sulfide nanoshell starting from a
cobalt nanocluster.
Time evolution of a hollow cobalt sulfide nanocrystal grown
from a cobalt nanocrystal via the nanoscale Kirkendall effect
Science 2004, 304, 711
NANOSCALE PATTERNING OF SHAKE-AND-BAKE
SOLID-STATE CHEMISTRY
MINIMIZING DIFFUSION LENGTHS
<x>  (2Dt)1/2 FOR RAPID AND
COMPLETE DIRECT REACTION
BETWEEN SOLID STATE MATERIALS
AT LOWEST T
Younan Xia
PDMS MASTER
Whitesides
Whitesides
PDMS MASTER
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Schematic illustration of the
procedure for casting PDMS
replicas from a master
having relief structures on its
surface.
The master is silanized and
made hydrophobic by
exposure to
CF3(CF2)6(CH2)2SiCl3 vapor
Each master can be used to
fabricate more than 50 PDMS
replicas.
Representative ranges of
values for h, d, and l are 0.2 20, 0.5 - 200, and 0.5 - 200 mm
respectively.
NANOSCALE PATTERNING OF SHAKE-AND-BAKE
SOLID-STATE CHEMISTRY
Younan Xia
NANOCALE PATTERNING OF SHAKE-AND-BAKE
SOLID-STATE CHEMISTRY
Co(NO3)2
Co3O4
Co
(A) Optical micrograph (dark field) of an ordered 2-D
array of nanoparticles of Co(NO3)2 that was
fabricated on a Si/SiO2 substrate by selective
dewetting from a 0.01 M nitrate solution in 2propanol. The surface was patterned with an array of
hydrophilic Si-SiO2 grids of 5 x 5 mm2 in area and
separated by 5 mm.
(B) An SEM image of the patterned array shown in
(A), after the nitrate had been decomposed into Co3O4
by heating the sample in air at 600 °C for 3 h. These
Co3O4 particles have a hemispherical shape (see the
inset for an oblique view).
(C) An AFM image (tapping mode) of the 2-D array
shown in (B), after it had been heated in a flow of
hydrogen gas at 400 °C for 2 h. These Co particles
were on average 460 nm in lateral dimensions and 230
nm in height.
NANOCALE PATTERNING OF SHAKE-AND-BAKE
SOLID-STATE CHEMISTRY
MgFe2O4
NiFe2O4
AFM image of an ordered 2-D array of
(A) MgFe2O4 and (B) NiFe2O4 that was
fabricated on the surface of a Si/SiO2
substrate by selective dewetting from the
2-propanol solution (0.02 M) that
contained a mixture of two nitrates [e.g.,
1:2 between Mg(NO3)2 and Fe(NO3)3].
The PDMS stamp contained an array of
parallel lines that were 2 mm in width
and separated by 2 mm. Citric acid was
also added to reduce the reaction
temperature between these two nitrate
solids in forming the ferrite.
Ferrite nanoparticles ~300 nm in lateral
dimensions and ~100 nm in height.
SIZE, DISPERSITY, SHAPE AND ORIENTATION
CONTROL OF NANOCRYSTAL SUPERLATTICE
• Shape is critical for command of structural, physical, chemical
properties of nanocrystals and their arrays
• Shape determines electrical, optical, magnetic behavior
• Magnetic anisotropy control of nanocrystals crucial for aligning easy
axis of magnetization of nanocrystal necessary for magnetic
applications like data storage
• Magnetic spinels test case for shape-controlled synthesis
• Fe(acac)3 + Mn(acac)2 + 1,2-hexadecanediol + oleic acid +
oleylamine + benzylether (high boiling solvent)  12 nm
MnFe2O4
• Size command by concentration of metal acetylacetonate precursors
• Shape control by amount of stabilizing surfactant capping agents
1,2-hexadecanediol, oleic acid, oleylamine – selective adsorption on
different crystal faces and growth of those facets
SIZE, DISPERSITY, SHAPE AND ORIENTATION
CONTROL OF NANOCRYSTAL SUPERLATTICE
• TEM low and high
resolution images of 12
nm MnFe2O4 cube and
polyhedral nanocrystal
superlattices
• Grown on Si(100)
substrate by EISA from
hexane dispersion
• Showing control over
size, dispersity, shape
and orientation
SIZE, DISPERSITY, SHAPE AND ORIENTATION
CONTROL OF NANOCRYSTAL SUPERLATTICE
• TEM and PXRD of 12 nm
MnFe2O4 cube and
polyhedral nanocrystal
superlattices
• Grown on Si(100)
substrate by EISA from
hexane dispersion
• Showing control over
size, dispersity, shape
and orientation
NUCLEATION OF PRODUCT PHASE
MgO ccp O(2-) with Mg(2+) diffusion
Al2O3 hcp O(2-) with Al(3+) diffusion
Nucleation at interface favored by structural
similarity of reagents and products
Minimal structural reorganization
Promotes nucleation and growth
Both MgO and MgAl2O4 similar O(2-) ccp
Spinel lattice nuclei matches MgO at interface
Essentially continuous across reaction interface
STRUCTURAL SIMILARITY OF REACTANTS AND PRODUCTS
PROMOTES NUCLEATION RATE AND PRODUCT GROWTH
FACTORS INFLUENCING CATION DIFFUSION RATES
IN SOLID STATE REACTIONS: MgO+Al2O3
• Lattice of oxide anions, mobile Mg(2+) and Al(3+)
cations
• Factors influencing cation diffusion rates
• Charge, mass and temperature
• Interstitial Frenkel versus substitutional Schottky
diffusion
• Depends on number and types of defects in reactant
and product phases
• Point, line, planar defects, grain boundaries
TOPOTACTIC AND EPITACTIC REACTIONS
• What about orientation effects on reactivity in the
bulk and surface regions of solids?
• Implies structural relationships between two
phases
• Topotactic reactions occur in bulk materials with
1D, 2D or 3-D structures - 1D TiS3, 2D MoO3, 3D
WO3
• Topotaxy more specific, require interfacial and bulk
crystalline structural similarity, lattice matching
TOPOTAXY: HOST-GUEST INCLUSION
1D- Tunnel Structures
-TiS3
Pivotal topotactic materials
properties for functional
utility in e.g., Li solid state
batteries, electrochromic
mirrors and windows, fuel
cell electrolytes, chemical
sensors, superconductors
2D- Layered Structures
3D-Frameworks
-Graphite
-TiS2
-KxMnO2
-FeOCl
-HxMoO3
-LiCoO2
- LiMn2O4
- WO3
TOPOTACTIC AND EPITACTIC REACTIONS
• Topotaxy: involves host-guest inclusion
• Requires available space or creates space in the
process of adsorption and injection
• Epitactic reactions occurs at interfaces, inherently
a 2-D phenomenon
• Epitactic reactions requires structural similarity in
2-D
EFFECT OF LATTICE MATCHING IN REACTIVITY OF
SOLIDS - EPITACTIC AND TOPOTACTIC REACTIONS
Lattice A unit cell dimension a
Missing atom - interfacial vacancy
Structural relationship between two
phases - topotaxy in bulk, epitaxy at
interfaces - epitaxy requires interfacial
structural similarity - lattice
matching <15% to tolerate oriented
nucleation and growth - otherwise
mismatch over large contact areas strained interfaces, missing atoms,
misfit dislocation
Lattice B unit cell dimension a’
EFFECT OF LATTICE MATCHING IN REACTIVITY OF
SOLIDS - EPITACTIC AND TOPOTACTIC REACTIONS
Lattice A unit cell dimension a
Missing atom - interfacial vacancy
Causes elastic strain at interface even lattice mismatch < 0.1% - slight
atom displacement from equilibrium
position in lattice - strain energy
reduced by misfit dislocation creates dangling bonds, localized
electronic states, charge carrier
scattering by defects, luminescence
quenching - reduces efficacy of
devices - visualized by TEM imaging
Lattice B unit cell dimension a’