Transcript SOLID-STATE MATERIALS SYNTHESIS METHODS

TOPOTACTIC SOLID-STATE SYNTHESIS
METHODS: HOST-GUEST INCLUSION CHEMISTRY
• Ion-exchange, injection, intercalation type synthesis
• Ways of modifying existing solid state structures while
maintaining the integrity of the overall structure
• Precursor structure
• Open structure or porous framework
• Ready diffusion of guest atoms, ions, organic molecules,
polymers, organometallics, coordination compounds,
nanoclusters, bio(macro)molecules into and out of the
structure of nanoporous crystals
TOPOTAXY: HOST-GUEST INCLUSION
1D- Tunnel Structures
-TiO2
-hWO3
-TiS3
Pivotal topotactic materials and
their properties for functional
utility in Li solid state battery
electrodes, electrochromic
mirrors, windows and displays,
fuel and solar cell electrolytes and
electrodes, chemical sensors,
superconductors, gas storage
2D- Layered Structures
3D-Frameworks
-zeolites
-MOFs
-LiMn2O4
-cWO3
-Graphite
-TiS2
-TiO2(B)
-KxMnO2
-FeOCl
-HxMoO3
-b alumina
-LixCoO2
TOPOTACTIC SOLID-STATE SYNTHESIS
METHODS: HOST-GUEST INCLUSION CHEMISTRY
• Penetration into interlamellar spaces: 2-D intercalation
• Into 1-D channel voids: 1-D injection
• Into cavity spaces: 3-D injection
• Classic materials for this kind of topotactic chemistry
• Zeolites, TiO2, WO3: channels, cavities
• Graphite, TiS2, NbSe2, MoO3: interlayer spaces
• Beta alumina: interlayer spaces, conduction planes
• Polyacetylene, NbSe3: inter chain channel spaces
TOPOTACTIC SOLID-STATE SYNTHESIS
METHODS: HOST-GUEST INCLUSION CHEMISTRY
• Ion exchange, ion-electron injection, atom, molecule,
coordination complex, cluster and polymer,
intercalation and occlusion, achievable by non-aqueous
and aqueous solution phase, gas phase and melt
techniques
• Chemical and electrochemical synthesis methods
• This type of topotactic solid state chemistry creates new
materials with novel properties, useful functions and
wide ranging applications and myriad technologies
GRAPHITE
out of plane pp orbitals - p/p* delocalized bands
A
B
sp2 in plane s bonding
VDW gap 3.35Å
C-C 1.41Å, BO 1.33
A
ABAB stacked
hexagonal graphite
Pristine graphite - filled p band - empty
p* band - narrow gap - semimetal
GRAPHITE INTERCALATION COMPOUNDS
4x1/4 K = 1
8x1 C = 8
C8K stoichiometry
G (s) + K (melt or vapor)  C8K (bronze)
C8K (vacuum, heat)  C24K  C36K  C48K  C60K
Staging, distinct phases, ordered guests, K  G CT
AAAA sheet stacking sequence
K nesting between parallel eclipsed hexagons,
Typical of many graphite H-G inclusion compounds
GRAPHITE INTERCALATION
ELECTRON DONORS AND ACCEPTORS
SALCAOs of the pp –pp type create the p valence and p*
conduction bands of graphite, very small band gap, essentially
metallic conductivity, single crystal conductivity in-plane 104
times that of out-of plane - thermal, electrical properties tuned
by degree of CB band filling or VB emptying
E
C
C8K electron transfer to
C2pp CB – metallic
reductive intercalation
p*
CB
p*
Eg
p
s
VB
p
s
E(F)
C8Br electron depletion
from C2pp VB – metallic
oxidative intercalation
p*
p
E(F)
s
N(E)
INTERCALATION REACTIONS OF GRAPHITE
Always Ask: Oxidative, Reductive or Charge Neutral?
• G (HF/F2/25oC)  C3.3F to C40F (white)
• intercalation via HF2- not F- - relative size, charge, ion and dipole,
polarizability effects - less strongly interacting - more facile diffusion
• G (HF/F2/450oC)  CF0.68 to CF (white)
• G (H2SO4 conc.)  C24(HSO4).2H2SO4 + H2
• G (FeCl3 vapor)  CnFeCl3
• G (Br2 vapor)  C8Br
PROPERTIES OF INTERCALATED GRAPHITE
• Structural planarity of layers often unaffected by
intercalation - bending of layers has been observed intercalation often reversible
• Modification of thermal and electrical conductivity behavior
by tuning degree of p*-CB filling or p-VB emptying
• Anisotropic properties of graphite intercalation systems
usually observed
• Layer spacing varies with nature of the guest and loading
• CF: 6.6 Å, C4F: 5.5 Å, C8F: 5.4 Å
BUTTON CELLS
LITHIUM-GRAPHITE FLUORIDE BATTERY
Composite CFx cathode
with C black particles to
enhance electrical
conductivity and
poly(vinylidenedifluoride)
PVDF binder to provide
mechanical stability
e
FLiF
Li+
Al contact
SS contact
Li anode
CFx/C/PVDF
cathode
Li+/PEO
BUTTON CELLS
LITHIUM-GRAPHITE FLUORIDE BATTERY
• Cell electrochemistry
• xLi + CFx  xLiF + C
• xLi  xLi+ + e• Cx+xF- + xLi+ + xe-  C + xLiF
Nominal cell voltage 2.7 V
• CFx safe storage of fluorine, intercalation of graphite by fluorine
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Millions of batteries sold yearly, first commercial Li battery, Panasonic
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Lightweight high energy density battery - cell requires stainless steel
electrode/lithium metal anode/Li+-PEO fast ion conductor/CFx intercalate acetylene black electrical conductor – poly(vinylidenedifluoride) mechanical
support cathode/aluminum charge collector electrode
C60-G INTERCALATING BUCKBALL INTO GRAPHITE
NEW HYDROGEN STORAGE MATERIAL???
• Thermally induced 600C
intercalation of C60 into G
• Hexagonal close packed C60
between graphene sheets
• Sieves H2 from larger N2
• Physisorbed H2 in intralayer
void spaces
• Rapid adsorption-desorption
• Dead capacity because of
volume occupied by C60
• Capacity possibly enhanced by
reducing filling fraction of C60
SURPRISE-SURPRISE
NATURE PHYSICS 2005, 1, 39
HIGH TC INTERCALATED SUPERCONDUCTING C6Y AND C6Ca
GRAPHITES – VP ITERCALATION OF Yb AND Ca INTO GRAPHITE
XRD shows every layer filled with Yb, stage 1, interlayer spacing 4.57 Å,
AA registered graphite layers and a-b offset triangular array Yb layers,
superconductivity mechanism under investigation
SYNTHESIS OF BORON AND NITROGEN
GRAPHITES - INTRALAYER DOPING
• New ways of modifying the properties of graphite
• Instead of tuning the degree of CB/VB filling with
electrons and holes using the traditional intercalation
methods focus on intralayer doping
• Put B or N into the graphite layers, deficient and rich
in carriers, enables intralayer doping with holes (VB)
and electrons (CB) respectively
• Big question delocalized
intraband or localized interband
dopant states???)
• Also provides access to a new intercalation chemistry
SYNTHESIS OF BC3
THEN PROVING IT IS SINGLE PHASE?
• Traditional heat and beat
• xB + yC (2350oC)  BCx
• Maximum 2.35 at% B incorporation in C
• Poor quality not well-defined materials
• New approach, soft chemistry, lower T, flow reaction, quartz tube
• 2BCl3 + C6H6 (800oC)  2BC3 (lustrous film on walls) + 6HCl
CHEMICAL AND PHYSICAL
CHARACTERIZATION OF BC3
• BC3 + 15/2F2  BF3 + 3CF4
• Fluorine burn technique
• BF3 : CF4 = 1 : 3
• Shows BC3 composition
• No evidence of precursors or intermediates
• Electron and Powder X-Ray Diffraction Analysis
• Shows graphite like interlayer reflections (00l)
CHEMICAL AND PHYSICAL
CHARACTERIZATION OF BC3
• 2BC3 (polycryst) + 3Cl2 (300oC)  6C (amorph) + 2BCl3
• C (cryst graphite) + Cl2 (300oC)  C (cryst graphite)
• This neat experiment proves B is truly a "chemical"
constituent of the graphite sheet and not an amorphous
component of a "physical" mixture with graphite
• Synthesis, PXRD structural analysis, chemical and
physical properties all indicate a graphite like structure
for BC3 with an ordered B, C arrangement in the layers
STRUCTURE OF BORON GRAPHITE BC3
Rietfeld PXRD Structure Refinement
4Cx1/4 + 2Cx1/2 + 10Cx1 = 12C
6Bx1/2 + 1Bx1 = 4B
Probable layer atomic arrangement with stoichiometry BC3
CHEMICAL AND PHYSICAL
CHARACTERIZATION OF BC3
• BC3 interlayer spacing similar to graphite
• Also similar to graphite-like BN made from thermolysis of
inorganic benzene - borazine B3N3H6 - thinking outside of
the box - F doping by using fluorinated borazine!!!
• Four probe basal plane resistivity on BC3 flakes
 s(BC3)AB ~ 1.1 s(G)AB, (greater than 2 x 104 ohm-1cm-1)
• Implies B effect is not the un-filling of VB to give a metal but
rather the creation of localized states in electronic band gap
making boron graphite behave like a substitution site doped
graphite maybe with a larger band gap
• Recall heteronuclear BN is a wide band gap insulator!!!
BOTTOM LINE - ELECTRONIC BAND
DESCRIPTION OF BC3
E
C
p*
CB
BC3 delocalized states
BC3 localized states
in C2pp VB
near C2pp VB
p*
p*
Eg
p
s
VB
p
s
E(F)
p
E(F)
s
N(E)
4-PROBE CONDUCTIVITY MEASUREMENTS
L
A
I = V1/R1
I
Rsample = V2/I
Rsample = (V2R1)/V1
r = Rsample (A/L)
V2
Constant
current
source
R1
s = 1/r
V1
Ohmeter
REPRESENTATIVE BC3 INTERCALATION CHEMISTRY
• BC3 + S2O6F2  (BC3)2SO3F Oxidative Intercalation
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Note: O2FSO--OSO2F, peroxydisulfuryl fluoride strong oxidizing agent, weak
peroxy-linkage easily reductively cleaved to stable fluorosulfonate anion 2SO3F-
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(BC3)2SO3F
Ic = 8.1 Å,
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BC3
Ic = 3-4 Å ,
(C7)SO3F
C
Ic = 7.73 Å,
Ic = 3.35 Å,
(BN)3SO3F
BN
Ic = 8.06 Å
Ic = 3.33 Å
• More Juicy Redox Intercalation Chemistry for BC3
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BC3 + Na+Naphthalide-/THF  (BC3)xNa (bronze, first stage, Ic ~ 4.3 Å)
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BC3 + Br2(l)  (BC3)15/4Br (deep blue)
INTERCALATION SYNTHESIS OF TRANSITION
METAL DICHALCOGENIDES
• Group IV, V, VI MS2 and MSe2 Compounds
• Layered structures
• Most studied is TiS2
• hcp S2• Ti4+ in Oh sites
• Van der Waals gap
INTERCALATION SYNTHESIS OF TRANSITION
METAL DICHALCOGENIDES
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Li+ intercalated between the layers
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Li+ resides in well-defined Td S4 interlayer sites
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Electrons injected into Ti4+ t2g CB states or localized
state in electronic bandgap
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LixTiS2 with tunable band filling and unfilling
• Localized xLi(I)xTi(III)(1x)Ti(IV)S2 mixed valence vs
delocalized xLiTi(IV-x)S2
electronic bonding models???
• Hopping semiconductor mixed valence
description xLi(I)xTi(III)(1-x)Ti(IV)S2
• VDW gap prized apart by 10%
ELECTRONIC DESCRIPTION OF LixTiS2
E
E
t2g Ti(IV) delocalized
t2g Ti(III) localized
S(-II) 3pp VB
N(E)
• DOS electronic band description LixTix(III)Ti(1-x) (IV)S2 mixed
valence localized t2g states (hopping semiconductor - Day and Robin Class
II) or LixTi (IV-x)S2 delocalized partially filled t2g band (metal - Day and
Robin Class III)
• Distinguished by s(T) temperature dependent electrical conductivity
(semiconductor not metal), optical detection of Ti(III)  Ti(IV)
intervalence charge transfer IVCT, and electron paramagnetic resonance
EPR detection of unpaired electron on Ti(III)
CHEMICAL SYNTHESIS OF LixTiS2
• xC4H9Li + TiS2 (hexane, N2/RT)  LixTiS2 + x/2C8H18
E
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E
t2g Ti(IV) delocalized
Filter, hexane wash
• 0x1
t2g Ti(III) localized
S(-II) 3pp VB
N(E)
• DOS electronic band description LixTix(III)Ti(1-x) (IV)S2
mixed valence localized t2g states (hopping semiconductor
- Day and Robin Class II) or LixTi (IV-x)S2 delocalized
partially filled t2g band (metal - Day and Robin Class III)
SEEING INTERCALATION - DIRECT
VISUALIZATION OPTICAL MICROSCOPY
Intercalating lithium - see the layers spread apart
ELECTROCHEMICAL SYNTHESIS OF LixTiS2
TiS2 + xLi+ + xe-  LixTiS2 AN ATTRACTIVE ENERGY STORAGE SYSTEM???
2.5V open circuit = (EF(Li)-EF(TiS2) - no
current drawn - energy density 4 x
Pb/H2SO4 battery of same weight
Controlled potential coulometry, voltage
controlled Li+ intercalation where x is
number of equivalents of charge passed
e-
Li metal anode: Li  Li+ +ePEO/Li(CF3SO3) polymer-salt solid
electrolyte or propylene carbonate/LiClO4
non aqueous electrolyte
Li+
PVDF(filler)/C(conductor)/TiS2/Pt(contact)
composite cathode – mechanical stability,
electronic and ionic conductivity:
TiS2 + xLi+ +xe-  LixTiS2
Li/TiS2 AN ATTRACTIVE ENERGY SOURCE BUT
MANY TECHNICAL OBSTACLES TO OVERCOME
• Technical problems to overcome with both the Li anode,
intercalation cathode and polymer-salt electrolyte
• Battery cycling causes Li dendritic growth at anode - need
other Li-based anode materials, Li-C composites, Li-Sn, LiSi alloys - also rocking chair LixMO2 configuration
• Mechanical deterioration at the cathode due to multiple
intercalation-deintercalation lattice volume expansioncontraction cycles
• Cause lifetime, corrosion, reactivity, and
kaboom safety hazards – challenge for
large scale electric car LSSB
LiCoO2
LixC6
ROCKING CHAIR LSSB
LiCoO2
TO AVOID Li DENDRITES
Li
HOW TO SYNTHESIZE A BETTER LSSB?
Improved Performance Cathode, Anode and Electrolyte
TEMPLATE SYNTHESIS OF NANOSCALE
BATTERY CATHODE MATERIALS
A BETTER BATTERY CATHODE USING
NANOSCALE MATERIALS - NANODIFFUSION
LENGTHS FOR Li+ DIFFUSIVE INTERCALATION
• Template synthesis is a versatile nanomaterial
fabrication method used to make monodisperse
nanoparticles of a variety of materials including
metals, semiconductors, carbons, and polymers.
• The template method has been used to prepare
nanostructured lithium-ion battery electrodes in
which nanofibers or nanotubes of the electrode
material protrude from an underlying currentcollector surface like the bristles of a brush.
• Nanochannel template made of Al2O3, Si, PC
• Nanostructured electrodes composed of C,
LiMn2O4, V2O5, Sn, TiO2 and TiS2
• Prepared by precursor synthesis, chemical vapor
deposition, sol-gel or melt infiltration
A BETTER BATTERY CATHODE USING
NANOSCALE MATERIALS - NANODIFFUSION
LENGTHS FOR Li+ DIFFUSIVE INTERCALATION
• In all cases, the nanostructured electrode
showed dramatically improved rate
capabilities relative to thin-film control
electrodes composed of the same material.
• The rate capabilities are improved because
the distance that Li must diffuse in the solid
state (the current- and power-limiting step in
Li-ion battery electrodes) is significantly
smaller in the nanostructured electrode.
• For example, in a nanofiber-based electrode,
the distance that Li must diffuse is restricted
to the radius of the fiber, which may be as
small as 50 nm.
A BETTER BATTERY CATHODE USING
NANOSCALE MATERIALS - NANODIFFUSION
LENGTHS FOR Li+ DIFFUSIVE INTERCALATION
• Beating mechanical stability problem of repeated
intercalation-deintercalation expansion-contraction
cycles
• In addition to improved rate capabilities, the
nanostructured electrodes do not suffer from poor
cyclability observed for conventional electrodes.
• This is because the absolute volume changes in the
nanofibers are small, and because of the brush-like
configuration, there is room to accommodate the
volume expansion around each nanofiber.
• Improved cycle life results show nanostructured
electrode can be driven through 1400
charge/discharge cycles without loss of capacity.
Ti(IV)-X- surface
coordinated anion
Li+ cation
nc-TiO2
PEO polymer chain
coordinated to Li+ cation
and surface Ti(IV)
Nanocrystal-LiX-PEO electrolytes
solid plasticisers for LSSB
Ti(IV)-O surface
coordinated oxygen of
PEO polymer chain
nc-TiO2
LiClO4-PEO-nc-TiO2
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LiClO4-PEO-ncTiO2 -high surface area nanocrystalline ceramic
Brnsted and Lewis acid-base sites - surface Ti(IV) coordination to O(CH2CH2)Surface Ti(IV) binding to counteranion XPolymer-particle crosslinking - no 60oC glass transition of PEO
ncTiO2 stabilizes glassy polymer state at RT
Domains of local polymer disorder at PEO-ncTiO2 interface
Optimal anchoring promotes local structural and dynamical modifications
High Li+ conductivity at RT
Excellent mechanical stability, improved stress-strain curves
Reduced reactivity with solid ncTiO2 compared to organic liquid plasticizer
Less cooperative PEO segmental motion with enhanced interfacial mobility of Li+
Transport number t(Li+), 0.3 pristine LiClO4-PEO, 0.6 in LiClO4-PEO-nc-TiO2
nc-CERAMIC OXIDES: SOLID PLASTICISERS IN
POLYMER-ELECTROLYTE LITHIUM BATERIES
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LiClO4 : PEO = 1 : 8-10 wt% ncTiO2 or Al2O3,
anchoring PEO oxygens and
counteranions to BrnstedLewis acid surface sites,
enhanced corrosion resistance of
electrodes,
better mechanical stability PEO,
higher Li+ conductivity &
transport number,
local disorder of polymer, loss of
Tg, stabilizes RT glassy state,
discards need for PEO-Li+
cooperative segmental motion
OTHER INTERCALATION SYNTHESES WITH TiS2
• Cu+, Ag+, H+, NH3, RNH2, Cp2Co, chemical, electrochemical
• Cobaltacene Cp2Co(II) especially interesting 19e guest
• [Cp2Co(III)]x+Tix3+Ti1-x4+S2 chemical-electronic description
consistent with structure, hopping SC, optical spectroscopy
Synthesis, Cp2CoCH3CN (solution)TiS2(s)
Co
Co
• Cp2Co almost spherical, temperature dependent 1H and 13C solid state
NMR shows Cp ring wizzing (lower T) and molecule tumbling dynamics
(higher T) with Cp2Co+ molecular axis orthogonal and parallel to layers,
dynamics yields activation energies for the different rotational processes
EXPLAINING THE MAXIMUM 3Ti: 1Co
STOICHIOMETRY IN (Cp2Co)0.3TiS2
Interleaved Cp2Co(+)
cations
Matching trigonal
symmetry of hcp
chalcogenide sheet
Maximum of third of
interlayer space filled
Geometrical and steric
requirements of packing
transverse oriented
metallocene in VDV gap
INTERCALATION ZOO
• Channel, layer and framework materials
• 1-D chains: TiO2 channels, (TiS3 [Ti(IV)S(2-)S2(2-)], NbSe3
[Nb(IV)Se(2-)Se2(2-)]), contain disulfide and diselenide units in
Oh building blocks to form chain
• 2-D layers: MS2, MSe2, NiPS3 [Ni2(P2S6), ABA CdI2 type packing,
alternating layers of octahedral NiS6 and trigonal P2S6 groupings
with S…S Van der Waals gap], FeOCl, V2O5.nH2O, MoO3, TiO2
(layered polymorph B – see Chimie Douce later)
• 3D framework: zeolites, WO3, Mo6S8, Mo6Se8 (Chevrel phases)
LixTiO2 – LITHIUM IONS INJECTED INTO CHANNELS AND ELECTRONS
INTO CONDUCTION BAND OF RUTILE CRYSTAL STRUCTURE
z
y
x
FACE BRIDGING OCTAHEDRAL TITANIUM
TRISULFIDE AND NIOBIUM TRISELENIDE
BUILDING BLOCKS FORM 1-D CHAINS
TiS3 = Ti(IV)S(2-)S2(2-)
intercalated cations like
Li(+) in channels
between chains to form
LixTiS3
1D high-purity TiS3 powder
is synthesized by direct stoichiometric
reaction of titanium and sulfur
(evacuated quartz-tube, 600 °C, 15 d)
and single crystals grown by VPT
with Br2 transport agent
Ti(IV) = S2(2-) =
S(2-) = Li(+) =
3-D OPEN FRAMEWORK TUNGSTEN OXIDE AND
TUNGSTEN OXIDE BRONZES MxWO3
W
O
c-WO3 = c-ReO3 structure type with
injected cation xM(q+) center of
cube and charge balancing xqeMxWO3 Perovskite structure type
M(q+) O CN = 12, O(2-) W CN = 2,
W(VI) O CN = 6
M
Unique 2-D layered structure of
MoO3
Chains of corner sharing
octahedral building blocks sharing
edges with two similar chains,
Creates corrugated MoO3 layers,
stacked to create interlayer VDW
space,
Three crystallographically distinct
oxygen sites, sheet stoichiometry
3x1/3 ( ) +2x1/2 ( )+1 ( )
MxMoO3
xM(q+) intercalated between
sheets with charge balancing xqe
ELECTROCHEMICAL OR CHEMICAL
SYNTHESIS OF MxWO3
• xNa+ + xe- + WO3  NaxWx5+W1-x6+O3
• xH+ + xe- + WO3  HxWx5+W1-x6+O3
• Injection of alkali metal cations generates Perovskite
structure types
• M+ oxygen coordination number 12, resides at center of cube
• H+ protonates oxygen framework, exists as MOH groups
SYNTHESIS DETAILS FOR Mx’MO3
WHERE M = Mo, W AND M’ = INJECTED PROTON
OR ALKALI OR ALKALINE EARTH CATION
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n BuLi/hexane
CHEMICAL
LiI/CH3CN
Zn/HCl/aqueous
Na2S2O4 aqueous alkaline sodium dithionite
S2O4(2-) + 4OH(-)  2SO3(2-) + 2H2O + 2e
Pt/H2
Topotactic ion-exchange of Mx’MO3 with M” cation
• Li/LiClO4/MO3
ELECTROCHEMICAL
• Cathodic reduction in aqueous acid electrolyte
• MO3 + H2SO4 (0.1M)  HxMO3
VPT GROWTH OF LARGE SINGLE CRYSTALS OF
MOLYBDENUM AND TUNGSTEN TRIOXIDE AND
CVD GROWTH OF LARGE AREA THIN FILMS
• VPT CRYSTAL GROWTH
• MO3 + 2Cl2 (700°C)  (800°C) MO2Cl2 + Cl2O
• CVD THIN FILM GROWTH
• M(CO)6 + 9/2O2 (500°C)  MO3 + 6CO2
MANY APPLICATIONS OF THIS M’xMO3
CHEMISTRY AND MATERIALS
• Electrochemical devices like pH sensors, electrochromic
displays, electrochromic energy saving windows, lithium solid
state battery cathodes, proton conducting solid electrolytes in
H2-O2 fuel cell
• Behave as low dopant mixed valance hopping semiconductors
• Behave as high dopant metals
• Electrical and optical properties best
understood by reference to simple DOS picture of
M’xMx5+M1-x6+O3
COLORING MOLYBDENUM TRIOXIDE WITH
PROTONS, MAKING IT ELECTRONICALLY, IONICALLY
CONDUCTIVE AND A SOLID BRNSTED ACID
Electronic band structure in HxMoO3 molybdenum oxide bronze, tuning color,
electronic conductivity, acidity with x
COLOR OF TUNGSTEN BRONZES, MxWO3
INTERVALENCE W(V) TO W(VI) CHARGE TRANSFER
IVCT
ELECTRONIC AND COLOR CHANGES BEST
UNDERSTOOD BY REFERENCE TO SIMPLE BAND
PICTURE OF NaxMox5+Mo1-x6+O3
• SEMICONDUCTOR TO METAL
TRANSITION ON DOPING MxMoO3
• MoO3: Band gap excitation from O2(2pp) VB to Mo6+ (5d) CB, LMCT in UV
region, wide band gap insulator
• NaxMox5+Mo1-x6+O3: Low doping level,
narrow band gap hopping
semiconductor, narrow localized Mo5+
(d1) VB, visible absorption, essentially
IVCT Mo5+ to Mo6+ absorption, mixed
valence hopping semiconductor
• NaxMox5+Mo1-x6+O3: High doping level,
partially filled valence band, narrow
delocalized Mo5+ (d1) VB, visible
absorption, IVCT Mo5+ to Mo6+ and
shows metallic reflectivity (optical
plasmon) and metallic conductivity
HxMoO3 TOPOTACTIC PROTON INSERTION
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Range of compositions: 0 < x < 2, MoO3 structure largely unaltered by reaction, four phases
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0.23 < x < 0.4
0.85 < x < 1.04
1.55 < x < 1.72
2.00 = x
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Similar lattice parameters by XRD, ND of HxMoO3 cf MoO3
orthorhombic
monoclinic
monoclinic
monoclinic
• MoO3 high resistivity semiconductor
• HxMoO3 insertion material SC to M transition
• HxMoO3 strong Brnsted acid: Mo-O(H)-Mo solid acid
catalysts
• HxMoO3 fast proton conductor: Mo-O(H)-Mo-O proton
oxygen site to site hopping – useful in solid electrolyte in H2O2 fuel cells
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What happens when single crystal immersed in Zn/HCl/H2O H(+) + e(-)?
H2-O2 FUEL CELL WITH PROTON CONDUCTING MEMBRANE
Proton conducting
membrane allows only the
protons to pass
Proton conducting
membrane
HxMoO3 TOPOTACTIC PROTON INSERTION
INTRALAYER PROTON DIFFUSION
1-D proton conduction along chains
Yellow transparent
Protons begin in basal plane
Moves from two edges along c-axis
INTERLAYER PROTON DIFFUSION
b-axis adjoining layers react
Orange transparent (semiconducting)
PROTON FILLING
Eventually proton diffusion complete and
entire crystal transformed Blue bronze
Consistent with structural, electrical and
optical data (metallic)
PROTON CONDUCTION PATHWAY IN HxMoO3
c-axis
PROTON CONDUCTION
PATHWAY IN HxMoO3
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Part of a HxMoO3 layer
Showing initial 1-D proton conduction pathway
Apical to triply bridging oxygen proton migration first
1H wide line NMR, PGSE NMR probes of structure and diffusion
Doubly to triply bridging oxygen proton migration pathway
Initial proton mobility along c-axis intralayer direction for x = 0.3
Subsequently along b-axis interlayer direction
Single protonation at x = 0.36, double protonation x = 1.7
More mobile protons higher loading D(300K) ~ 10-11 vs 10-9 cm2s-1
Proton-proton repulsion
ION EXCHANGE SOLID STATE SYNTHESIS
• Requirements: anionic open channel, layer or
framework structure
• Replacement of some or all of charge balancing
cations by protons or simple or complex cations
• Classic cation exchangers are zeolites, clays,
beta-alumina, molybdenum and tungsten oxide
bronzes, lithium intercalated metal
dichalcogenides
BETA ALUMINA
• High T synthesis of beta-alumina:
• (1+x)/2Na2O + 5.5Al2O3  Na1+xAl11O17+x/2
• Structural reminders (x ~ 0.2):
• Na2O: Antifluorite ccp Na+, O2- in Td sites
• Al2O3: Corundum hcp O2-, Al3+ in 2/3 Oh sites
• Na1+xAl11O17+x/2 defect Spinel, O2- vacancies in conduction
plane, controlled by x ~ 0.2, Spinel blocks 9Å thick, bridging
oxygen columns, mobile Na+ cations in conduction plane
• 2-D fast sodium ion conductor
Rigid Al-O-Al
column spacers
Na(+) conduction
plane
0.9 nm
Na1+xAl11O17+x/2
defect spinel
blocks
3/4 O(2-) missing in
conduction plane
Spinel blocks, ccp layers of O(2-)
Every 5th. layer has 3/4 O(2-) vacant, defect spinel
4 ccp layers have 1/2Oh, 1/8Td Al( 3+) cation sites
Blocks cemented by rigid Al-O-Al spacers
Na(+) mobile in 5th open conduction plane
Centrosymmetric layer sequence in Na1+xAl11O17+x/2
(ABCA)B(ACBA)C(ABCA)B(ACBA)
GETTING BETWEEN THE SHEETS OF THE BETA
ALUMINA FAST SODIUM CATION FAST ION
CONDUCTOR: LIVING IN THE FAST LANE
0.9 nm Spinel block
Al-O-Al column
spacers in conduction
plane
Oxide wall of
conduction plane
Mobile sodium cations
ION EXCHANGE IN Na1+xAl11O17+x/2
Thermodynamic and
kinetic considerations
Mass, size and charge
considerations
Lattice energy controls
stability of ionexchanged materials
Cation diffusion,
polarizability effects
control rate of ionexchange
MELT ION-EXCHANGE OF CRYSTALS
• Equilibria between beta-alumina and MNO3 and MCl
melts, 300-350oC
• Extent of exchange depends on time, T, melt composition
• Monovalents: Li+, K+, Rb+, Ag+, Cu+, Tl+, NH4+, In+, Ga+,
NO+, H3O+
• Higher valent cations: Ca2+, Eu3+, Pb2+
• Higher T melts required for exchange of higher valency
cations, strong cation binding, slower cation diffusion,
600-800oC typical
MELT ION-EXCHANGE OF CRYSTALS
• Charge-balance requirements:
• 2Na+ for 1Ca2+, 3Na+ for 1La3+
• Controlled partial exchange by
control of melt composition:
• qNaNO3 : (1-q)AgNO3
• Na1+x-yAgyAl11O17+x/2
KINETICS AND THERMODYNAMICS OF SOLID
STATE ION EXCHANGE
• Kinetics of Ion-Exchange
•
•
Controlled by ionic mobility of the cation
Mass, charge, radius, temperature, solvent, solid state structural properties
• Thermodynamics, Extent of Ion-Exchange
•
•
•
•
•
Ion-exchange equilibrium for cations
Binding activities between melt and crystal phases
Site preferences
Binding energetics, lattice energies
Charge : radius ratios