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Chapter 9

Chapter 9. Shake and Bake?. The Solid-State Synthesis. Classification of Solids. Single Crystal Preferred for characterization of structure and properties. Polycrystalline Powder (Highly crystalline)

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Chapter 9

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  1. Chapter 9 Shake and Bake? The Solid-State Synthesis

  2. Classification of Solids • Single Crystal Preferred for characterization of structure and properties. • Polycrystalline Powder (Highly crystalline) Used for characterization when single crystal can not be easily obtained, preferred for industrial production and certain applications. • Polycrystalline Powder (Large Surface Area) Desirable for further reactivity and certain applications such as catalysis and electrode materials • Amorphous (Glass) No long range translational order. • Thin Film Widespread use in microelectronics, telecommunications, optical applications, coatings, etc.

  3. Idealized reaction mixture composed of grains of MgO and Al2O3

  4. Fishes to birds

  5. Nucleation of MgAl2O4 spinel on MgO and Al2O3

  6. Spinel product layer separating MgO and Al2O3 reactant grains

  7. Aspects of Solid-Solid Reactions Conventional solid state synthesis techniques involve heating mixtures of two or more solids to form a solid phase product. Unlike gas phase and solution reactions, the limiting factor in solid-solid reactions is usually diffusion.

  8. Rates of Reaction are controlled by three factors: • The area of contact between reacting solids • The rate of diffusion • The rate of nucleation of the product phase

  9. The area of contact between reacting solids • To maximize the contact between reactants we want to use starting reagents with large surface area. Consider the numbers for a 1 cm3 volume of a reactant • Edge Length = 1 cm# of Crystallites = 1 Surface Area = 6 cm2 • Edge Length = 10 mm# of Crystallites = 109Surface Area = 6 x 103 cm2 • Edge Length = 100Å# of Crystallites = 1018Surface Area = 6 x 106 cm2 • Pelletize to encourage intimate contact between crystallites.

  10. The rate of diffusion Two ways to increase the rate of diffusion are to • Increase temperature • Introduce defects by starting with reagents that decompose prior to or during reaction, such as carbonates or nitrates.

  11. The rate of nucleation of the product phase • We can maximize the rate of nucleation by using reactants with crystal structures similar to that of the product (topotactic and epitactic reactions).

  12. What are the consequences of high reaction temperatures? • It can be difficult to incorporate ions that readily form volatile species (i.e. Ag+) • It is not possible to access low temperature, metastable (kinetically stabilized) products. • High (cation) oxidation states are often unstable at high temperature, due to the thermodynamics of the following reaction: 2MOn (s)  2MOn-1(s) + O2(g) Due to the presence of a gaseous product (O2), the products are favored by entropy, and the entropy contribution to the free energy become increasingly important as the temperature increases. DG = DH -TDS

  13. Steps in Conventional Solid State Synthesis 1. Select appropriate starting materials • Fine grain powders to maximize surface area • Reactive starting reagents are better than inert • Well defined compositions 2. Weigh out starting materials 3. Mix starting materials together • Agate mortar and pestle (organic solvent optional) • Ball Mill (Especially for large preps > 20g)

  14. Steps in Conventional Solid State Synthesis 4. Pelletize • Enhances intimate contact of reactants • Minimizes contact with the crucible • Organic binder may be used to help keep pellet together 5. Select sample container Reactivity, strength, cost, ductility all important a) Ceramic refractories (crucibles and boats) • Al2O3 1950oC $30/(20 ml) • ZrO2/Y2O3 2000oC $94/(10 ml) b) Precious Metals (crucibles, boats and tubes) • Pt 1770° C $500/(10 ml) • Au 1063° C $340/(10 ml) • Ag 960° C $ 43/(10 ml) • Ir 2450° C $930/(10 ml) c) Sealed Tubes • SiO2- Quartz • Au, Ag, Pt • Nb, Ta, Mo, W

  15. Steps in Conventional Solid State Synthesis 6. Heat • Factors influencing choice of temperature include Tamman’s rule (2/3mp) and potential for volatilization • Initial heating cycle to lower temperature can help to prevent spillage and volatilization • Atmosphere is also critical • Oxides (Oxidizing Conditions) – Air, O2, Low Temps • Oxides (Reducing Conditions) – H2/Ar, CO/CO2, High T • Nitrides – NH3 or Inert (N2, Ar, etc.) • Sulfides – H2S • Sealed tube reactions, Vacuum furnaces 7) Grind product and analyze (x-ray powder diffraction) 8) If reaction incomplete return to step 4 and repeat.

  16. Take the synthesis of Sr2CrTaO6 as example 1) Possible starting reagents • Sr Metal – Hard to handle, prone to oxidation • SrO - Picks up CO2 & water, mp = 2430° C • Sr(NO3)2 – mp = 570° C, may pick up some water • SrCO3 – decomposes to SrO at 1370° C • Ta Metal – mp = 2996° C • Ta2O5 – mp = 1800° C • Cr Metal – Hard to handle, prone to oxidation • Cr2O3 – mp = 2435° C • Cr(NO3)3.‧nH2O – mp = 60° C, composition inexact

  17. 2) Weigh out starting reagents • To make 5.04 g of Sr2CrTaO6 (FW = 504.2 g/mol; 0.01 mol) to complete the reaction: 4SrCO3 + Ta2O5 + Cr2O3 2Sr2CrTaO6 + 4CO2 you need: • SrCO3 2.9526 g (0.02 mol) • Ta2O5 2.2095 g (0.005 mol) • Cr2O3 0.7600 g (0.005 mol) 3) Grind in a mortar and pestle for 5-15 minutes, then press a pellet

  18. 4) Applying Tamman’s rule (T = 2/3 mp) to each of the reagents: • SrCO3 SrO 1370° C (1643 K) • SrO mp = 2700 K 2/3 mp = 1527° C • Ta2O5 mp = 2070 K  2/3 mp = 1107° C • Cr2O3 mp = 2710 K 2/3 mp = 1532° C Although you may get a complete reaction by heating to 1150° C, in practice there will still be a fair amount of unreacted Cr2O3. Therefore, to obtain a complete reaction it is best to heat to 1500-1600° C. The initial heating cycle should be slow, or a preliminary fire at 1400° C should be used to prevent the SrCO3 from violently decomposing and spilling out of the crucible.

  19. Precursor Routes • Approach : Decrease diffusion distances through intimate mixing of cations. • Advantages : Lower reaction temps, possibly stabilize metastable phases, eliminate intermediate impurity phases, produce products with small crystallites/high surface area. • Disadvantages : Reagents are more difficult to work with, can be hard to control exact stoichiometry in certain cases, sometimes it is not possible to find compatible reagents (for example ions such as Ta5+ and Nb5+ immediately hydrolyze and precipitate in aqueous solution). • Methods : With the exception of using mixed cation reactants, all precursor routes involve the following steps: (a)Mixing the starting reagents together in solution. (b)Removal of the solvent, leaving behind an amorphous or nano-crystaline mixture of cations and one or more of the following anions: acetate, citrate, hydroxide, oxalate, alkoxide, etc. (c)Heat the resulting gel or powder to induce reaction to the desired product.

  20. Coprecipitation Synthesis of ZnFe2O4 • Mix the oxalates of zinc and iron together in water in a 1:1 ratio. Heat to evaporate off the water, as the amount of H2O decreases a mixed Zn/Fe acetate (probably hydrated) precipitates out. • Fe2((COO)2)3 + Zn(COO)2 Fe2Zn((COO)2)5‧xH2O • After most of the water is gone, filter off the precipitate and calcine it (1000° C). • Fe2Zn((COO)2)5 ZnFe2O4 + 4CO + 4CO2 This method is easy and effective when it works. It is not suitable when : 1) Reactants of comparable water solubility cannot be found. 2)The precipitation rates of the reactants is markedly different. These limitations make this route unpractical for many combinations of ions. Furthermore, accurate stoichiometric ratios may not always be maintained.

  21. Chimie Douce (Soft Chemistry) • Approach : Chimie Douce reactions are carried out under moderate conditions (typically T < 500° C). Chimie Douce reactions are topotactic, meaning that structural elements of the reactants are preserved in the product, but the composition changes. • Advantages : Chimie Douce Methods are very useful for the following applications: • Modifying the electronic structure of solids (doping) • Design of new metastable compounds (structural motif can be selected by choice of precursor, may have unusual properties) • Preparing reactive and/or high surface area materials used in heterogeneous catalysis, batteries and sensors • Disadvantages :First of all, one must find the appropriate precursor in order to carry out chemie douce. Secondly, metastable products are often unstable in applications where high temperatures are used or single xtals are needed

  22. Intercalation Chemistry • Involves inserting ions into an existing structure, this leads to a reduction (cations inserted) or an oxidation (anions inserted) of the host. • Typically carried out on layered materials (strong covalent bonding within layers, weak van der Waals type bonding between layers, i.e. graphite, clays, dicalchogenides, etc.). Performed via electrochemistry or via chemical reagents as in the n-butyl Li technique. Examples : TiS2 + nBu-Li  LiTiS2 b-ZrNCl + Naph-Li b-LixZrNCl

  23. Structures of graphite

  24. De-intercalation • The reverse of intercalation, also performed using either electrochemical methods or with reactive chemical species Examples : • NiMo3S4 Mo3S4 (Wash with HNO3) • In2Mo6S6 + 6HCl (g)  Mo6S6 + 2InCl3 (g) + 3H2 (g) This approach can often lead to new phases (polymorphs) of previously known compounds • CuTi2S4 cubic TiS2 • KCrSe2 layered CrSe2 • Li2FeS2 FeS2

  25. Dehydration By removing water and/or hydroxide groups from a compound, you can often perform redox chemistry and maintain a structural framework not accessible using conventional synthesis approaches Examples : • Ti4O7(OH)2‧nH2O  TiO2 (B) (500° C) • 2KTi4O8(OH) ‧nH2O  K2Ti8O17 (500° C)

  26. Ion Exchange Exchange charge compensating, ionically bonded cations (easiest for monovalent cations) Examples : • LiNbWO6 + H3O+ HNbWO6 + Li+ • Cubic-KSbO3 + Na+ Cubic-NaSbO3 + K+

  27. Hydrothermal (solvothermal) Synthesisand Crystal Growth Reaction takes place in superheated water, in a closed reaction vessel called a hydrothermal bomb (150 < T < 500° C; 100 < P < 3000 kbar). Seed crystals and a temperature gradient can be used for growing crystals Particularly common approach to synthesis of zeolites Example : • 6CaO + 6SiO2Ca6Si6O17(OH)2 (150-350° C) OH- can be used as mineralizer to enhance crystallinity Mineralizer effect: Precipitation ←→ Dissolution mechanism enhances crystallization

  28. Pressure-temperature relations for water at constant volume Hydrothermal Synthesis of Crystals", Robert A. Laudise (Bell Lab) Chemical And Engineering News, Vol. 65 (39), 30, 1987 Quartz crystal

  29. Sol-gel Synthesis Sol gel is a colloidal suspension that can be gelled to form a solid. In a typical sol-gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, then the particles condense in a new phase, the gel, in which a solid macromolecule is immersed in a solvent. Scientists have used it to produce the world’s lightest materials and some of its toughest ceramics.

  30. Hydrolysis and Condensation

  31. Sol-Gel Synthesis of Metastable ScMnO3 (metastable: 吃軟不吃硬) Begin by dissolving Sc2O3 and MnCO3, separately, in heated aqueous solutions of formic acid to form the formate salts: • Sc2O3 + 6HCOOH 2Sc(HCOO)3 + 3H2O • MnCO3 + 2HCOOH + 2H2O Mn(HCOO)2‧2H2O + H2CO3 • Addition of Sc(HCOO)3 and Mn(COOH)2‧2H2O to melted citric acid monohydrate results in the formation of a (Sc,Mn) citrate polymer. • Heat to 180° C  Removal of excess water and organics • Heat to 450° C  Formation of an amorphous oxide product • Heat to 690° C  Formation of crystalline ScMnO3 Direct reaction of the formates at 700° C simply gives the a mixture of the binary oxides: • 2Sc(HCOO)3 + 2Mn(COOH)2 ‧ 2H2O  Sc2O3 + Mn2O3 + 5CO2 + 2H2O + H2

  32. Single Crystal Growth • Structure determination and intrinsic property measurements are preferably, sometimes exclusively, carried out on single crystals. • For certain applications, most notably those which rely on optical and/or electronic properties (laser crystals, semiconductors, etc.), single crystals are necessary. Phosphate Laser Glass Nd-Doped KTiOAsO4 Neodymium-doped Gadolinium Gallium Garnet

  33. Slow cooling of the melt • With congruently melting materials (those which maintain the same composition on melting) one simply melts a mixture of the desired composition then cools slowly (typically 2-10° C/hr) through the melting point. • More difficult with incongruently melting materials, knowledge of the phase diagram is needed. • Often times the phase diagram is not known, consequently there is no guarantee that crystals will have the intended stoichiometry. • Molten salt fluxes are often used to facilitate crystal growth in systems where melting points are very high and/or incongruent melting occurs. • Crystals grown in this way are often rather small, thus this method is frequently used in research, but usually not appropriate for applications where large crystals are needed.

  34. Czochralski method A seed crystal is attached to a rod, which is rotated slowly. The seed crystal is dipped into a melt held at a temperature slightly above the melting point. A temperature gradient is set up by cooling the rod and slowly withdrawing it from the melt (the surrounding atmosphere is cooler than the melt) Decreasing the speed with which the crystal is pulled from the melt, increases the quality of the crystals (fewer defects) but decreases the growth rate. The advantage of the Czochralski method is that large single crystals can be grown, thus it used extensively in the semiconductor industry.

  35. Artistic Ge Si Art and Science

  36. Stockbarger, Bridgman and zone melting method Stockbarger : Crucible moves, hot zone stationary Bridgman : Hot zone moves, crucible stationary CaF2 grown by Bridgman method

  37. Chemical Vapor Transport A polycrystalline sample, A, and a transporting species, B, are sealed together inside a tube. Upon heating the transporting species reacts with the sample to produce a gaseous species AB. When AB reaches the other end of the tube, which is held at a different temperature, it decomposes and redeposits A.

  38. Chemical Vapor Transport • If formation of AB is endothermic (rxn ← , as T ↓ ) crystals are grown in the cold end of the tube. A (powder) + B (g)  AB (g) (hot end)AB (g)  A (xtal) + B (g) (cold end) • If formation of AB is exothermic (rxn → , as T ↓ ), crystals are grown in the hot end of the tube. A (powder) + B (g)  AB (g) (cold end)AB (g)  A (xtal) + B (g) (hot end) • Typical transporting agents include: I2, Br2, Cl2, HCl, NH4Cl, H2, H2O, TeCl4, AlCl3, CO, S2 Temperature gradient is typically created and controlled using a two-zone furnace.

  39. Chemical Vapor Transport Examples : • Growth of Fe3O4 crystals Fe3O4 (s) + 8HCl (g)  FeCl2 (g) + FeCl3 (g) + 4H2O (g)(Endothermic) • Growth of ZrNCl crystals ZrNCl (s) + 3HCl (g)  ZrCl4 (g) + NH3 (g)(Exothermic) • Growth of Ca2SnO4 crystals SnO2 (s) + CO  SnO (g) + CO2 (g)SnO (g) + CO2 (g) + 2CaO (s)  Ca2SnO4 (s) + CO (g) ZnO needles grown by CVT

  40. Thin Film Deposition All deposition methods involve growing a film whose thickness can vary from tens of angstroms to several millimeters, on a preexisting substrate. The methods vary in their methods of delivering the reactants/product to the substrate and in their operating conditions. Classification of Thin Films (a) single crystals(b) epitaxial(c) polycrystalline(d) amorphous

  41. Uses of Thin Films (a) microelectronic devices(b) telecommunication devices(c) wear resistant coatings(d) decorative coatings(e) optical coatings (windows, solar cells, etc.)(f) sensors(g) catalysts

  42. Chemical Vapor Deposition • Similar to chemical vapor transport, involves one or more gas phase species which react on a solid surface (substrate) to deposit a solid flim. • Typically, the reaction is initiated by heating the substrate. Other mechanisms of supplying the activation energy necessary to initiate reactions include: laser CVD, photo CVD, and plasma enhanced CVD.

  43. Chemical Vapor Deposition Examples WCl6 (g) + 3H2 (g)  W (s) + 6HCl (g)SiH4 (g) + O2 (g)  SiO2 (s) + 2H2 (g)6TiCl4 (g) + 8NH3 (g)  6TiN (s) + 24HCl (g) + N2 (g)

  44. Metal-Organic Chemical Vapor Deposition Single source precursor molecules for MOCVD The use of organometallic precursors as gas phase species (MOCVD) can result in significant reduction of the substrate temperature.

  45. Sputtering • Sputtering is similar in several respects to laser ablation. The primary difference is that ion bombardment is used instead of a laser pulse to displace atoms or clusters of atoms from the target. • The ions which bombard the target are produced in a plasma (gaseous collection of ionized and neutral species) discharge. The plasma is created by application of a large electric field, which partially ionizes the neutral gas (Ar, He, N2, etc.) present in the chamber. • Plasma – Neutral species, electrons, positive ions • DC Electric Field – Metallic (Conductive) TargetsRF Electric Field – Insulating and conductive Targets

  46. Vacuum Evaporation Vacuum evaporation consists of three steps: (a) Transition of the solid/liquid source to the gas phase (resistive heating, flash evap., e-beam)(b) Transport of the vapor to the substrate(c) Condensation of the vapor on the substrate (deposition)

  47. Cathode sputtering and vacuum evaporation equipments for thin film deposition cathode sputtering vacuum evaporation

  48. Verneuil method (also called flame fusion) The principle of the process involves melting a finely powdered substance using an oxyhydrogen flame, and crystallising the melted droplets into a boule. Verneuil furnace, where finely ground purified alumina and chromium oxide were melted by a flame of at least 2000 °C (3,600 °F), and recrystallised on a support below the flame, creating a large crystal.

  49. High pressure polymorphism of some simple solids • Dense packing of ions (higher cation coordination numbers) • (b) Higher cation oxidation states • (c) Higher symmetry

  50. Molten Salt Fluxes Why solvents? Solubilize reactants  Enhance diffusion  Reduce reaction temperature Synthesis in a solvent is the common approach to synthesis of organic and organometallic compounds. This approach is not extensively used in solid state syntheses, because many inorganic solids are not soluble in water or organic solvents. However, molten salts turn out to be good solvents for many ionic-covalent extended solids.

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