1 Reactions in liquid state 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 Solution Methods 2 Precursor Coprecipitation Method Coprecipitation applicable to nitrates, acetates, oxalates, hydroxides, alkoxides, beta-diketonates Requires: similar salt/complex 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 Coprecipitation Method LDH = layered double hydroxides (hydrotalcites) Mg6Al2(OH)16CO3.4H2O Mg(NO3)2ꞏ6H2O + Al(NO3)3ꞏ9H2O Aqueous solutions Low supersaturation Addition of a NaOH solution pH during precipitation kept constant at 9.0 Suspension aged at 373 K for 15 h w/stirring Centrifugation, washing, drying Brucite Mg(OH)2 4 Precursor Coprecipitation Method Spinels Oxalates: Zn(CO2)2 / Fe2[(CO2)2]3/H2O, 1 : 1 mixing, H2O evaporation, salts coprecipitation Solid-solution mixing on atomic scale, filtration, calcination in air Zn(CO2)2 + Fe2[(CO2)2]3  ZnFe2O4 + 4 CO + 4 CO2 Al2O3 Bayer Process bauxite + NaOH / pressure  Al(OH)4  (+ insoluble Fe(OH)3, TiO2, SiO2) Al(OH)4  + CO2 (g)  Al(OH)3 2 Al(OH)3  -Al2O3 + 3 H2O BaTiO3 BaCl2 + TiOCl2 + 2 H2C2O4 + 4 H2O + Ln3+ (dopants)   BaTiO(C2O4)2.4H2O + 4 HCl Filtration, washing, drying, calcination @ 730 C 5 Coprecipitation Method Spinel Al(NO3)3 + Mg(NO3)2 + H2O Freeze-drying gives amorphous mixture Calcination @ 800 C (!!! low T) Mixture-Al(NO3)3 / Mg(NO3)2  random-MgAl2O4 + 6 NOx + (103x) O2 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 (YSZ) ZrSiO4 (zircon) + NaOH  Na2ZrO3 + Na2SiO3 Na2ZrO3 + Na2SiO3 + HCl  ZrOCl2 + SiO2 ZrOCl2 + NaOH + YCl3  Zr(OH)4 / Y(OH)3  nano-Y/ZrO2 (YSZ) 6 Coprecipitation Method High-Tc Superconductors La3+ + Ba2+ + Cu2+ + H2C2O4  ppt mixed oxalates  La1.85Ba0.15CuO4 @ 1373 K Magnetic Garnets Y(NO3)3 + Gd(NO3)3 + FeCl3 + NaOH  ppt mixed M(OH)3  YxGd3-xFe5O12 Firing @ 900 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 - tunable magnetic materials Oxalate Coprecipitation 7 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 - FeC2O4ꞏ2H2O and CoC2O4ꞏ2H2O have temperature-dependent polymorphisms: monoclinic α (90 C) and orthorhombic β (25 C), MnC2O4ꞏ2H2O forms only monoclinic The final step: Solid state reaction of Mn1/3Fe1/3Co1/3(C2O4) 3 2H2O and LiH2PO4 Oxalate Coprecipitation 8 UO2 +ThO2 U1-xThxO2 • Solutions of UIV (0.5 M, obtained by electroreduction of UO2(NO3)2 solution in 4 M HNO3 containing 0.5 M of hydrazine) • ThIV (1.9 M in 8 M HNO3) • Adding a stoichiometric amount of 1 M solution of oxalic acid • precipitate was washed with water and dried • Hydrothermal decomposition of • under argon control of ox state of U 4+ vs 6+ addition of hydrazine • different solubilities of MC2O4 2H2O • control of atmosphere – U4+ get easily oxidized to U6+ • control of temperature and aging time 18 h at 250 C in an autoclave • Spark plasma sintering 9 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 water evaporation and 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 10 Pechini and Citrate Gel Method HOCH2CH2OH Chelation Complexation-coordination polymers Polyesterification polycondensation 11 Pechini and Citrate Gel Method BaTiO3 by conventional powder method at 1200 C Ba2+ + Ti(OiPr)4 + citric acid  gel  BaTiO3 at 650 C ScMnO3 Sc2O3 + 6 HCOOH  2 Sc(HCOO)3 + 3 H2O MnCO3 + 2 HCOOH  Mn(HCOO)2 + CO2 + H2O Added to citric acid, heating, water removal Gel formation Calcination @ 690 C gives perovskite ScMnO3 Without citric acid only a mixture of Sc2O3 and Mn2O3 is formed 12 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 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 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 14 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, cell volume, band gap, transition temperatures, …… Any property P of a solid-solution member A1-xBx (x = 01) is the atom fraction weighted average of the end-members The composition of the A1-xBx phase 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 in Solid-Solutions 15 Vegard’s Law in CdSe1-xSx 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 16 Vegard’s Law in La1-xCexCrO3 Any property P of a solid-solution member is the atom fraction weighted average of the end-members: P(A1-xBx) = x P(B) + (1  x) P(A) 17 Flux Method Molten salts (inert or reactive) Metals, metal oxides MNO3, MOH, (M = alkali metal) M2Qx (M = alkali metal, Q = S, Se, Te) FLINAK: LiF-NaF-KF FLIBE: LiF-BeF2 • 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 direct powder method • Products finely divided solids, powders of high surface area (SA) • Slow cooling to grow single crystals • Separation of water insoluble product from a water soluble flux • Incorporation of the molten salt ions in products prevented by using salts with ions of much different sizes than the ones in the product (PbZrO3 in a B2O3 flux) 18 FLINAK and FLIBE Melting point 454 °C Eutectic (46.5 - 11.5 - 42 mol %) A coolant in the molten salt reactors FLINAK: LiF-NaF-KF FLIBE: LiF-BeF2 Melting point of 360 °C Eutectic (50% BeF2) A neutron moderator and coolant in the nuclear reactors 19 Flux Method Lux-Flood formalism oxide = strong base acid = oxide acceptor A + OB  AO + B base = oxide donor Zr(SO4)2 + eut. (Li/K)NO3  ZrO2 @700 K Zr(SO4)2 + eut. (Li/K)NO2  ZrO2 @540 K ZrOCl2 + eut. (Na/K)NO3  ZrO2 amorph. @540 K calcine  t-ZrO2 ZrOCl2 + YCl3 + eut. (Na/K)NO3  ZrO2 (YSZ) @720 K BaCO3 + SrCO3 + TiO2 + eut. (Na/K)OH  cubic-Ba0.75 Sr0.25TiO3 @570 K 20 Flux Method Zeolites Fly ash (aluminosilicates) + NaOH, NH4F, NaNO3  sodalite, cancrinite Metal phosphates NH4H2PO4 + (Na/K)NO3 + M(NO3)2  (Na/K)MPO4 Mixed-metal oxides 4 SrCO3 + Al2O3 + Ta2O5  Sr2AlTaO6 900 C in SrCl2 flux 1400 C required for a direct reaction Chalcogenides K2Tex + Cu  K2Cu5Te5 K2Tex reactive flux, 350 C 21 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 22 N N N N (CH2)n NN N N N HO OH Ionic Liquids Organic cations (containing N, P) Ammonium, imodazolium, pyridinum,…. Inorganic anions: Cl-, AlCl4 -, Al2Cl7 -, Al3Cl10 -, PF6 -, SnCl3 -, BCl3 -, BF4 -, NO3 -, OSO2CF3 - (triflate), CH3C6H4SO3 -, N(SO2CF3)2 -, PO4 3- 23 Ionic Liquids (IL) 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 > 4 V (not oxidized or reduced) • IL immiscible with organic solvents • Hydrophobic IL immiscible with water 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] Autosolvolysis Keq = 1016 to 1017 at 40 ºC 2 [AlCl4] ⇄ [Al2Cl7] + Cl 2 [Al2Cl7] ⇄ [Al3Cl10] + [AlCl4] Acidic: excess of AlCl3 x(AlCl3) > 0.5 Basic: excess of Cl x(AlCl3) < 0.5 Neutral: [AlCl4] x(AlCl3) = 0.5 X1 = Cl X10 = [Al3Cl10] X4 = [AlCl4] X13 = [Al4Cl13] X7 = [Al2Cl7] X6 = Al2Cl6 Equilibria in Halogenoaluminate(III) IL 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 Superacidity Scale, H0 27 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 28 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 29 Density and Viscosity of Ionic Liquids The density of IL decreases as the bulkiness of the organic cation increases: The viscosity of IL depends on van der Waals interactions and H-bonding [BMIM] 30 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 [BMIM] Water-soluble: Br, CF3COO, CF3SO3 Water-immiscilble: PF6, (CF3SO2)2N IL are 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 31 Applications of Ionic Liquids Electrodeposition 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 32 Applications of Ionic Liquids Olefin polymerization Ethene + TiCl4 + AlEtCl2 in acidic IL  Polyethylene Ethene + Cp2TiCl2 + Al2Me3Cl3 in acidic IL  Polyethylene Cp2TiCl2 + [cation]+[Al2Cl7] ⇄ [Cp2TiCl] + + [cation]+ + 2 [AlCl4] Olefin hydrogenation Cyclohexene + H2 + [RhCl(PPh3)3] (Wilkinson’s catalyst)  Cyclohexne 33 Applications of Ionic Liquids Biphasic solvent systems: IL / organic solvent 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