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 FLINAK: LiF-NaF-KF 18 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 19 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 20 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 21 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 22 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 23 Ionic Liquids 24 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 25 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 26 Equilibria in Halogenoaluminate(III) IL Equilibria in IL X1 = Cl X4 = [AlCl4] X7 = [Al2Cl7] X10 = [Al3Cl10] X13 = [Al4Cl13] X6 = Al2Cl6 27 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 28 Superacidity 29 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 30 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 31 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 32 Melting Point of Ionic Liquids 33 Density of Ionic Liquids The density of IL decreases as the bulkiness of the organic cation increases: 34 Viscosity of Ionic Liquids The viscosity of IL depends on: van der Waals interactions H-bonding 35 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 36 Solubility in/of Ionic Liquids 37 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 38 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 39 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)