Precursor Methods Goals - decrease diffusion paths, shorten reaction times and 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 1 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 2 Coprecipitation Method Spinels oxalates: Zn(C02)2/Fe2[(C02)2]3/H20 1 : 1 mixing, H20 evaporation, salts coprecipitation Solid-solution mixing on atomic scale, filter, calcine in air Zn(C02)2 + Fe2[(C02)2]3 ZnFe204 + 4C0 + 4C02 Al203 Bayer Process Na0H, p C02 . x 1500 °C . . bauxite -^ Al(0H)4 -». Al(0H)3 1 JUU ► a-Al203 i Fe(0H)3, Ti02, Si02 BaTi03 BaCl2 + Ti0Cl2 + 2 H2C204 + 4 H20 + Ln dopants ► BaTi0(C204)2.4H20 + 4 HCl filtration, washing, drying, calcination @ 730 °C 3 Coprecipitation Method 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 0._ HCl ZrSiO4(zircon) + NaOH -r* Na2ZrO3 + Na2SiO3 -► ZrOCl2 -► Zr(OH)4 / Y(OH)3 ——-—► nano-Y/ZrO2 2 v 74 v 73 calcination 2 4 Coprecipitation Method High-Tc Superconductors 3 2 2 1373 K La + + Ba + + Cu + + H2C2O4 —► ppt -► LaL85Ba015CuO4 Magnetic garnets, tunable magnetic materials Y(NO3)3 + Gd(NO3)3 + FeCl3 + NaOH YxGd3-xFe5O12 Firing @ 900 oC, 18-24 hrs, pellets, regrinding, repelletizing, repeated firings, removes REFeO3 perovskite impurity Isomorphous replacement of Y for Gd 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 5 Pechini and Citrate Gel Method Aqueous solution of metal ions i—cooh ho--cooh 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 6 Pechini and Citrate Gel Method BaTi03 by conventional powder method at 1200 °C Ba2+ + Ti(0iPr)4 + citric acid at 650 °C Sc203 + 6 HCOOH -► 2Sc(HC00)3 + 3 H20 MnC03 + 2 HC00H -► Mn(HC00)2 + C02 + H20 added to citric acid, water removal, calcination @ 690 °C gives ScMn03 without citric acid only mixture of Sc203 and Mn203 is formed 7 Double Salt Precursors 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 Chromite spinel Precursor Ignition T, oC 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 Single Source Precursor Compounds containing desired elements in a proper stoichiometric ratio Easy chemical pathway for ligand removal M^O-Si-O \ M O H2O + (M-O-SiO3) 7v x 9 Vegard's Law Vegard law behavior: 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 will be given by a linear dependence of lattice parameter on composition: a(A1-xBx) = x a(B) + (1-x) a(A) 10 Vegard's Law c(CdSe1-xSx) = x c(CdS) + (1-x) c(CdSe) o 10 20 3d 40 K *0 to s& ůd 100 MOLE PERCEříT Cd5t Jfl Cd^Sift^) 11 VegarcPs Law P (YxCe!-xRhIn5) = x P (YxCe!-xRhIn5) + (1-x) P (YxCe!-xRhIn5) Any property P of a solid-solution member is the atom fraction weighted average of the end-members Tetragonal lattice constant a as a function of Y concentration x for the Ce1-xYxRhIn5 system 0 O.Z HA O.B 0.9 1 Y concentration (mol) VegarcPs Law A linear relationship exists between the concentration of the substitute element and the size of the lattice parameters The direction of the linear relationship, increasing or decreasing, depends upon the system being analyzed As the concentration of Y is increased, lattice constant a decreases, implying the cell is contracting along the a axis 13 Vegard's Law Vegard law behavior: P (YxGd3-xFe5012) = x/3 P (Y3Fe5012) + (3-x)/3 P (Gd3Fe5012) Any property P of a solid-solution member is the atom fraction weighted average of the end-members 14 Vegard's Law Tunable magnetic properties by tuning the x value in the binary garnet Y Gd3 Fe5O12 x 3-x 5 12 3 Td Fe3+ sites, 5 UPEs 2 Oh Fe3+ sites, 5UPEs Ferrimagnetically coupled material, oppositely aligned electron spins on the Td and Oh Fe3+ magnetic sublattices Counting spins Y3Fe5O12 ferrimagnetic at low T: 3 x 5 - 2 x 5 = 5UPEs Counting spins Gd3Fe5O12 ferrimagnetic at low T: 3 x 7 - 3 x 5 + 2 x 5 = 16 UPEs YxGd3-xFe5O12 creates a tunable magnetic garnet that is strongly temperature and composition dependent, applications in permanent magnets, magnetic recording media, magnetic bubble memories and so forth, similar concepts apply to magnetic spinels 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) Flux Method Lux-Flood formalism oxide = strong base acid = oxide acceptor A + OB —► AO + B base = oxide donor 700 K Zr(SO4)2 + eut. (Li/K)NO3 -► ZrO2 540 K Zr(SO4)2 + eut. (Li/K)NO2 -► ZrO2 520 K ZrOCl2 + eut. (Na/K)NO3-► ZrO2 amorph. —► t- ZrO2 720 K ZrOCl2 + YCl3 + eut. (Na/K)NO3 ► ZrO2 BaCO3 + SrCO3 + TiO2 + eut. (Na/K)OH 570 K _^ cubiC-Ba075 Sr025TiO3 18 Flux Method 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 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 e- cathode TiO2 pellet cathodic reduction Ti4+ + 4 e- — Ti 20 Ionic Liquids Organic cations (containing N, P) Inorganic anions: Cl-, AlCl4-, Al2Cl7-, Al3Cl10-, PF6-, SnCl3-, BCl3-, BF4-, NO3-, OSO2CF3- (triflate), CH3C6H4SO3-, N(SO2CF3)2-, PO43- 21 Ionic Liquids 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 22 Ionic Liquids 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 ±5 R+ [AlCl4]- 2 [AlCl4]- ±4 [Al2Cl7]- + Cl- autosolvolysis Keq = 10-16 to 10-17 at 40 °C 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 25 Equilibria in Halogenoaluminate(III) IL 26 Halogenoaluminate(III) Ionic Liquids 2 [AlCl4]- ±+ [Al2Cl7]- + Cl- autosolvolysis Keq = 10-16 to 10-17 at 40 °C 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]- ±4 M+ + 2 [AlCl4]- buffered IL B + M+ + [AlCl4]- ±+ MCl + B-AlCl3 27 Superacidity 28 Superacidic [EMIM]Cl/AlCl3/HCl I = not protonated II = slightly protonated III and IV = 10-20 % V = 75-90% VI-VIII = nearly completely IX and X = completely log Kb in HF 29 Ionic Liquids Completely inorganic ionic liquids 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(TiWnO39)2] 253.0 Na5[CrTiWuO39] 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 30 Melting Point of Ionic Liquids 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 R mp/^C Me Cl 125 Et Cl S7 n-Bu Cl 65 Et NO.7 j8 Et A1C14 7 Et BF4 ö Et CFjSOj -9 Et -3 Et CFjC02 -14 n-Bu CFjSO^ 16 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 MeEtlmCl at 120 °C BiCl3, SrCl2, CaCl2, CuCl2 dissolve well Constituent BiCl3 SrCl2 CaCl2 CuCl2 Concentration 0.068 0.50 0.18 0.050 (mol kg-1 MeEtlmCl) Substrate Al —1.72 V vs the Ag/Ag+ reference electrode 37 Applications of Ionic Liquids Biphasic solvent systems Preparation of aerogels 2 HCOOH + Si(OMe)4 -► ag-SiÜ2 + 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 38 Applications of Ionic Liquids Olefin polymerization Ethene + TiCl4 + AlEtCl2 in acidic IL Ethene + Cp2TiCl2 + Al2Me3Cl3 in acidic IL Cp2TiCl2 + [cation]+[Al2Cl7]_ ±4 [Cp2TiCl] + + [cation]+ + 2 [AlCl4]- Olefin hydrogenation Cyclohexene + H2 + [RhCl(PPh3)3] (Wilkinson's catalyst) 39