1
Goals :
• decrease diffusion paths
• shorten reaction times
• decrease reaction temperatures
Intimate mixing of components in solution, precipitation, filtration,
washing, drying, calcination
 High degree of homogenization
 Large contact area
 Reduction of diffusion distances
 Faster reaction rates
 Lower reaction temperatures
 Metastable phases, smaller grain size, larger surface area
 Shaping to fibers, films, nanoparticles
Precursor Methods
2
Coprecipitation Method
Coprecipitation applicable to nitrates, acetates, oxalates, hydroxides,
alkoxides, beta-diketonates
Requires: similar salt solubilities
similar precipitation rates
no supersaturation
Washing: water, organic solvents
Drying: evaporation
azeotropic distillation
freeze-drying
Disadvantage: difficult to prepare high purity, accurate stoichiometric
phases if solubilities do not match
3
Spinels
oxalates: Zn(CO2)2/Fe2[(CO2)2]3/H2O 1 : 1 mixing, H2O evaporation,
salts coprecipitation
Solid-solution mixing on atomic scale, filter, calcine in air
Zn(CO2)2 + Fe2[(CO2)2]3  ZnFe2O4 + 4CO + 4CO2
Al2O3 Bayer Process
bauxite Al(OH)4
Al(OH)3
-Al2O3
Fe(OH)3, TiO2, SiO2
BaTiO3
BaCl2 + TiOCl2 + 2 H2C2O4 + 4 H2O + Ln dopants
BaTiO(C2O4)2.4H2O + 4 HCl
filtration, washing, drying, calcination @ 730 C
NaOH, p CO2 1500 C
Coprecipitation Method
4
Spinel
Al(NO3)3 + Mg(NO3)2 + H2O freeze-drying gives amorphous
mixture, calcination @ 800 C !!! low T
Mg(NO3)2 + 2 Al(NO3)3 MgAl2O4 + 6 NOx + (10-3x)O2
random
Ruby
Ion exchange
Al(NO3)3 + Cr(NO3)3 Al(OH)3 + Cr(OH)3 sol
freeze drying gives solid (Al/Cr)(OH)3 @ LN2 temperature, 5 Pa
anealing @ 950 C for 2.5 h gives solid solution Al2-xCrxO3
Zirconia
ZrSiO4(zircon) + NaOH Na2ZrO3 + Na2SiO3
ZrOCl2 Zr(OH)4 / Y(OH)3 nano-Y/ZrO2
HCl
OH,
YCl3 azeot. dist.
calcination
Coprecipitation Method
5
High-Tc Superconductors
La3+
+ Ba2+
+ Cu2+
+ H2C2O4 ppt La1.85Ba0.15CuO4
Magnetic garnets, tunable magnetic materials
Y(NO3)3 + Gd(NO3)3 + FeCl3 + NaOH  YxGd3-xFe5O12
Firing @ 900 o
C, 18-24 hrs, pellets, regrinding, repelletizing,
repeated firings, removes REFeO3 perovskite impurity
Isomorphous replacement of Y3+
for Gd3+
on dodecahedral sites,
solid solution, similar rare earth ionic radii
complete family accessible, 0 < x < 3, 2Fe3+
Oh sites, 3Fe3+
Td sites,
3RE3+
dodecahedral sites
1373 K
Coprecipitation Method
Oxalate Coprecipitation
6
LiMPO4 (M = Mn, Fe, Co, or Ni)
• olivine structure
• new cathode materials for lithium rechargeable batteries
• multicomponent olivine cathode materials LiMn1/3Fe1/3Co1/3PO4
Mn1/3Fe1/3Co1/3(C2O4) 3 2H2O
stoichiometric, homogeneously mixed transition metal oxalate precursor
the differences in chemical behavior of Fe, Co, and Mn ions
• control of pH - different solubilities of MC2O4 2H2O
• control of atmosphere - Fe2+ get easily oxidized to Fe3+
• control of temperature and aging time - FeC2O4·2H2O and CoC2O4·2H2O
have temperature-dependent polymorphisms: monoclinic α (90 C) and
orthorhombic β (25 C), MnC2O4·2H2O forms only monoclinic
Solid state reaction of Mn1/3Fe1/3Co1/3(C2O4) 3 2H2O and LiH2PO4
7
COOH
COOH
COOH
HO
Pechini and Citrate Gel Method
Aqueous solution of metal ions
Chelate formation with citric acid
Polyesterification with polyfunctional alcohol on heating
Further heating leads to resin, transparent glassy gel
calcination provides oxide powder
Control of stoichiometry by initial reagent ratio
Complex compositions, mixture of metal ions
Good homogeneity, mixing at the molecular level
Low firing temperatures
8
Pechini and Citrate Gel Method
HOCH2CH2OH
Chelation
Complexation-coordination polymers
Polyesterification
polycondensation
9
BaTiO3
by conventional powder method at 1200 C
Ba2+
+ Ti(Oi
Pr)4 + citric acid at 650 C
Sc2O3 + 6 HCOOH 2Sc(HCOO)3 + 3 H2O
MnCO3 + 2 HCOOH Mn(HCOO)2 + CO2 + H2O
added to citric acid, water removal, calcination @ 690 C gives
ScMnO3
without citric acid only mixture of Sc2O3 and Mn2O3 is formed
Pechini and Citrate Gel Method
10
Chromite spinel Precursor Ignition T, o
C
MgCr2O4 (NH4)2Mg(CrO4)2.6H2O 1100-1200
NiCr2O4 (NH4)2Ni(CrO4)2.6H2O 1100
MnCr2O4 MnCr2O7.4C5H5N 1100
CoCr2O4 CoCr2O7.4C5H5N 1200
CuCr2O4 (NH4)2Cu(CrO4)2.2NH3 700-800
ZnCr2O4 (NH4)2Zn(CrO4)2. 2NH3 1400
FeCr2O4 (NH4)2Fe(CrO4)2 1150
Double salts of known and controlled stoichiometry such as:
Ni3Fe6(CH3COO)17O3(OH).12Py
Burn off organics 200-300 oC, then 1000 oC in air for 2-3 days
Product highly crystalline phase pure NiFe2O4 spinel
Good way to make chromite spinels, important tunable magnetic
materials
Juggling the electronic-magnetic properties of the Oh and Td ions in
the spinel lattice
Double Salt Precursors
Double Salt Precursors
11
12
M
O Si O
O
O
H2O + (M-O-SiO3) x
Single Source Precursors
Known phases in Cr-P system: Cr3P, Cr2P, Cr2P7, CrP, CrP2, CrP4
• Compounds containing desired elements in a proper stoichiometric ratio
• Easy chemical pathway for ligand removal
CrP
Thermolysis
180 °C for 1 h
Hexadecylamine (HDA)
Oleic acid (OlA)
Mesitylene
13
Vegard law behavior:
A linear relationship exists between the concentration of the substitute
element and a property of a solid-solution, e.g. the size of the lattice
parameters.
Any property P of a solid-solution member is the atom fraction weighted
average of the end-members
The composition of the A1-xBx alloy can be calculated from Vegard’s law
The lattice parameter of a solid solution alloy a will be given by a linear
dependence of lattice parameter on composition:
a(A1-xBx) = x a(B) + (1-x) a(A)
Vegard’s Law
14
Vegard’s Law
c(CdSe1-xSx) = x c(CdS) + (1-x) c(CdSe)
Anion radius
S2 1.84 Å
Se2 1.98 Å
a hexagonal wurzite structure
a cubic zinc blende
a high pressure form with the NaCl structure
15
La1-xCexCrO3
16
Flux Method
Molten salts (inert or reactive), oxides, metals
MNO3, MOH, (M = alkali metal)
FLINAK: LiF-NaF-KF
M2Qx (M = alkali metal, Q = S, Se, Te)
molten salts ionic, low mp, eutectics, completely ionized
act as solvents or reactants, T = 250-550 C
enhanced diffusion, reduced reaction temperatures in comparison with
powder method
products finely divided solids, high surface area (SA)
slow cooling to grow crystals
separation of water insoluble product from a water soluble flux
incorporation of the molten salt ions in product prevented by using salts
with ions of much different sizes than the ones in the product
(PbZrO3 in a B2O3 flux)
17
Lux-Flood formalism
oxide = strong base
acid = oxide acceptor A + OB AO + B
base = oxide donor
Zr(SO4)2 + eut. (Li/K)NO3 ZrO2
Zr(SO4)2 + eut. (Li/K)NO2 ZrO2
ZrOCl2 + eut. (Na/K)NO3 ZrO2 amorph. t- ZrO2
ZrOCl2 + YCl3 + eut. (Na/K)NO3 ZrO2
BaCO3 + SrCO3 + TiO2 + eut. (Na/K)OH
cubic-Ba0.75 Sr0.25TiO3
700 K
540 K
520 K
720 K
570 K
Flux Method
18
fly ash (aluminosilicates)
NaOH, NH4F, NaNO3 zeolites (sodalite, cancrinite)
NH4H2PO4 + (Na/K)NO3 + M(NO3)2 (Na/K)MPO4
4 SrCO3 + Al2O3 + Ta2O5 Sr2AlTaO6
900 C in SrCl2 flux
1400 C required for a direct reaction
K2Tex + Cu K2Cu5Te5 K2Tex reactive flux, 350 C
Flux Method
19
Flux Method
Electrolysis in molten salts
Reduction of TiO2 pellets to Ti sponge in a CaCl2 melt at 950 C
O2- dissolves in CaCl2, diffuses to the graphite anode
insulating TiO2  TiO2-x conductive
graphite anode
anodic oxidation 2 O2-  O2 + 4 ecathode
TiO2 pellet
cathodic reduction Ti4+ + 4 e-  Ti
20
Organic cations (containing N, P)
Inorganic anions: Cl,
AlCl4
,
Al2Cl7
,
Al3Cl10
,
PF6
,
SnCl3
,
BCl3
-
,
BF4
,
NO3
,
OSO2CF3
(triflate),
CH3C6H4SO3
,
N(SO2CF3)2
,
PO4
3N
N
N N
(CH2
)n
NN
N
N N
HO OH
Ionic Liquids
21
Oldest known (1914) : EtNH3
+
NO3
mp
12 C
Liquids at room temperature or low mp
Thermal operating range from 40 C to 400 C
Higly polar, noncoordinating, completely ionized
Nonvolatile – no detectable vapor pressure
Nonflamable, nonexplosive, nonoxidizing, high thermal stability
Electrochemical window > 4V (not oxidized or reduced)
Immiscible with organic solvents
Hydrophobic IL immiscible with water
Ionic Liquids
22
Ionic Liquids
23
Synthesis of Ionic Liquids
NR3 + RCl  [NR4]+ Cl
Aluminates
[NR4]+ Cl + AlCl3  [NR4]+ [AlCl4]
Metal halide elimination
[NR4]+ Cl + MA  MCl + [NR4]+ A
Reaction with an acid
[NR4]+ Cl + HA  HCl + [NR4]+ A
Ion exchange
[NR4]+ Cl + Ion exchanger A  [NR4]+ A
24
Halogenoaluminate(III) Ionic Liquids
The most widely studied class of IL
High sensitivity to moisture – handling under vacuum
or inert atmosphere in glass/teflon
RCl + AlCl3  R+ [AlCl4]
2 [AlCl4]  [Al2Cl7] + Cl autosolvolysis Keq = 1016 to 1017 at 40 ºC
2 [Al2Cl7]  [Al3Cl10] + [AlCl4]
Acidic: excess of AlCl3 as [Al2Cl7] x(AlCl3) > 0.5
Basic: excess of Cl x(AlCl3) < 0.5
Neutral: [AlCl4] x(AlCl3) = 0.5
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Equilibria in Halogenoaluminate(III) IL
Equilibria in IL
X1 = Cl
X4 = [AlCl4]
X7 = [Al2Cl7]
X10 = [Al3Cl10]
X13 = [Al4Cl13]
X6 = Al2Cl6
26
Halogenoaluminate(III) Ionic Liquids
2 [AlCl4]  [Al2Cl7] + Cl autosolvolysis Keq = 1016 to 1017 at 40 ºC
Acidic IL with an excess of AlCl3
HCl + [Al2Cl7]  H+ + 2 [AlCl4]
Proton extremely poorly solvated = high reactivity
Superacid [EMIM]Cl/AlCl3/HCl H0 = -19 (HSO3F: H0 = -15)
Latent acidity
MCl + [Al2Cl7]  M+ + 2 [AlCl4] buffered IL
B + M+ + [AlCl4]  MCl + B-AlCl3
27
Superacidity
28
Superacidic [EMIM]Cl/AlCl3/HCl
log Kb in HF
I = not protonated
II = slightly protonated
III and IV = 10-20 %
V = 75-90%
VI-VIII = nearly completely
IX and X = completely
29
Compound mp (K) Compound mp (K)
Na13[La(TiW11O39)2] 253.0 Na13[Tm(TiW11O39)2] 260.2
Na13[Ce(TiW11O39)2] 263.0 Na13[Yb(TiW11O39)2] 267.2
Na13[Pr(TiW11O39)2] 253.0 Na5[CrTiW11O39] 261.5
Na13[Sm(TiW11O39)2] 256.0 Na5[MnTiW11O39] 253.0
Na13[Gd(TiW11O39)2] 265.1 Na5[FeTiW11O39] 257.6
Na13[Dy(TiW11O39)2] 265.2 Na6[ZnTiW11O39] 257.4
Na13[Er(TiW11O39)2] 261.0
Completely inorganic ionic liquids
Ionic Liquids
30
Melting Point of Ionic Liquids
Phase diagram of [EMIM]Cl/AlCl3
Melting point is influenced by:
Cation – low symmetry, weak imtermolecular interactions, good distribution of charge
Anion – increasing size leads to lower mp
Composition – Phase diagram
31
Melting Point of Ionic Liquids
32
Density of Ionic Liquids
The density of IL decreases as the bulkiness of the organic cation increases:
33
Viscosity of Ionic Liquids
The viscosity of IL depends on:
van der Waals interactions
H-bonding
34
Solubility in/of Ionic Liquids
Variation of the alkyl group
Increasing nonpolar character of the cation increases solubility of
nonpolar solutes.
Water solubility depends on the anion
water-soluble [BMIM] Br, CF3COO, CF3SO3
Water-immiscilble [BMIM] PF6 (CF3SO2)2N
IL miscible with organic solvent IF their dielectric constant is above a
certain limit given by the cation/anion combination
Polarity by E(T)(30) scale
[EtNH3][NO3] 0.95 between CF3CH2OH and water
[BMIM] PF6 as methanol
35
Solubility in/of Ionic Liquids
36
Applications of Ionic Liquids
Electrodeposition of metals and alloys (also nanoscopic)
Al, CoAlx, CuAlx, FeAlx, AlTix
Semiconductors Si, Ge, GaAs, InSb, CdTe
Electrodeposition of a Bi-Sr-Ca-Cu alloy (precursor to SC oxides)
Melt of MeEtImCl at 120 ºC
BiCl3, SrCl2, CaCl2, CuCl2 dissolve well
Constituent BiCl3 SrCl2 CaCl2 CuCl2
Concentration 0.068 0.50 0.18 0.050
(mol kg1 MeEtImCl)
Substrate Al
1.72 V vs the Ag/Ag+ reference electrode
37
Biphasic solvent systems
Preparation of aerogels
2 HCOOH + Si(OMe)4 ag-SiO2 + 2 MeOH + 2 HCOOMe
Natural gas sweetening (H2S, CO2 removal)
Electrolytes in batteries or solar cells
Dissolving spent nuclear fuel (U4+
oxidized to U6+
)
Extraction
Enyzme activity
Applications of Ionic Liquids
38
Applications of Ionic Liquids
Olefin polymerization
Ethene + TiCl4 + AlEtCl2 in acidic IL
Ethene + Cp2TiCl2 + Al2Me3Cl3 in acidic IL
Cp2TiCl2 + [cation]+[Al2Cl7]  [Cp2TiCl] + + [cation]+ + 2 [AlCl4]
Olefin hydrogenation
Cyclohexene + H2 + [RhCl(PPh3)3] (Wilkinson’s catalyst)