Materials Synthesis Solid-state synthesis: Ceramic method (aka heat & beat or shake & bake ) (generally): stoichiometric reaction of powders (elements, binary compounds, ternary compounds, etc.) at high temperature in air, under vacuum, or in inert or reactive gases. 1200 o C in air e.g., 2CaCO 3 + Fe 2 O 3 Ca 2 Fe 2 O 5 + 2CO 2(g) 2
Advantages: Simplicity (anyone can do it!) Shake & Bake: pros But can be complicated temperature profile:» ramp rate, dwell times, annealing temperature Atmosphere:» In flowing gas (N 2 /Ar/Ar+H 2 ); in evacuated quartz tube; in welded shut metal casing under inert atmosphere cooling (particularly for oxides)» Many structures are only stable at high OR low temperatures and oxygen stoichiometry can vary widely depending on cooling conditions» Can cool slowly in furnace under controlled conditions; in air (fast cool); in H 2 O (faster cooling; needs sealed reaction vessel); in liquid N 2 (fastest cooling) 3
Shake & Bake: cons Disadvantages: It s soooooo slow solid state diffusion limited Can limit problem if one or more reactants melts or forms a vapour below/at the reaction temperature A lot of energy may be required to break the bonds of the precursor High temperatures needed (can be expensive) Purification can be difficult 4
Shake & Bake: Considerations Cooling of material Environment (air, vacuum, etc.) Packing of material (pellet vs. free powder) Reactions are diffusion limited 5
Slow cooling of melts Crystallization techniques Chemical vapour transport (CVT) Flux growth Czochralski method Slow cooling of melts Used to form large single crystals Heat above melting point of material and slowly cool (cooling rate can be fractions of a degree per hour) Negative: melting point can be too high to access Positive: easy and can yield very large crystals Stibnite (Sb 2 S 3 ); mp = 550 o C 6
Chemical vapour transport (CVT) Charge Transport Hf(Si 0.5 As 0.5 )As Reactants T 1 T 2 Requirements: transport agent, thermal gradient, sealed reaction tube Product (crystals) Transport agents: I 2(g), Cl 2(g), HCl (g), TeBr 4(g) Technique used for single crystal growth, purification, synthesis of new materials What happens? Reactants form a gas phase intermediate with transport agent 7
Other crystallization techniques: flux growth Flux growth synthesis: Flux: inert material with low melting point Reactants must be soluble in flux Allows for reactants to intimately mix and order Slow cooling enables growth of larger crystals Flux removed by being dissolved (acid, water, ) or separated using centrifuge (at temperatures possibly above 1000 o C!!) like-dissolves-like : low melting point metals (e.g., Sn, Pb) used for intermetalic/covalent compounds while ionic compounds (e.g., Li 2 MoO 4 ) used for oxides (e.g, ZrSiO 4 ) 8
Other crystallization techniques: Czochralski and floating zone methods Si (single crystal) LaB 6 9
Atomic Layer-by-Layer Deposition Chemical Vapour Deposition (CVD): Highly volatile metal precursors decompose on heated/illuminated surface to give metal atoms and gaseous byproducts i.e. Ga(CH 3 ) 3 + AsH 3 GaAs(s) + 3CH 4 Disadvantages: Expensive and very toxic chemicals, clean-room conditions needed Molecular Beam Epitaxy (MBE): Similar to CVD but with less volatile substances -- beam of molecules formed by heating a compound in an oven with small hole in top - substrate placed just above opening Atomic metals (I.e. Ga, In, Al) can be used 10
Thin films Annealing forms quaternary compound at the interface D. C. Johnson, et. al. Chem. Mater. 22, 2010, 2750. G.D. Wilk, R.M. Wallace, Appl. Phys. Lett. 2000, 76, 112. 11
Hydrolysis of Metal Alkoxides: Sol-Gel Chemistry Hydrolysis Condensation Olation Oxolation Overall: M(OR) 4 + 4H 2 O 4ROH + MO 2 Reactivity: Si(OR) 4 << Sn(OR) 4 ~ Ti(OR) 4 < Zr(OR) 4 ~ Ce(OR) 4 Hydrolysis ratio h (h = [H 2 O]/[M(OR) n ] allows to control the extent of hydrolysis Silicon alkoxides -- acid (H + ) or base (OH - ) catalyst needed for hydrolysis step To avoid water loss (which can affect degree of hydrolysis, often done under inert atmosphere in system) 12
Sol-Gel Chemistry Increased hydrolysis/condensation Solvent, byproducts, etc. trapped If too dense, may precipitate (force out solvent) 13
Sol-Gel Chemistry Xerogel - conventional drying -- pore structure collapse due to surface tension Aerogel - extraction of solvent, leaves porous structure behind (i.e. supercritical CO 2 drying) 14
Sol-gel chemistry: not just for amorphous materials Sol-gel often used to make glasses (e.g., (HfO 2 ) x (SiO 2 ) 1-x, a high-k dielectric) Important step is annealing to remove organic residue Method produces well mixed, generally amorphous materials High T heating can yield powders of very pure, highly ordered/ crystalline materials High level of atomic mixing allows you to get over the diffusion limited hump Often can produce the wanted material at temperatures lower than those required when using the ceramic method 15
Annealing of sol-gel synthesized materials: homogeneous compounds Torres-Martinez, L.M.; Hernandez, A.; Luna-Urzua, J.C.; J. Mater. Online, 2005, DOI: 10.2240/azojomo0142 16
Annealing of sol-gel synthesized materials: heterogeneous mixtures Forms crystalline ZrO in a silica matrix (some ordered SiO 2 also forming) 17
How to Make Nanoparticles: bottom-up approach Arrested Nucleation and Growth nucleation, followed by passivation, and controlled growth passivation by organic ligands such as thiols for metal nanoparticles (Au, Ag) or trioctylphosphine oxide (TOPO) for CdSe nanoparticles helps prevent aggregation Want rapid nucleation of particles at time = zero, and no subsequent nucleation after this time For metal nanoparticle formation: add large excess of strong reducing agent (such as NaBH 4 ) to metal salts in the presence of stabilizer. 18
How to Make Nanoparticles From: B. Quinn, Nanocrystal Synthesis and Fabrication 19
Template synthesis 20
Top-down nanoparticle synthesis Turning large particles (µm) into much smaller particles (nm) Ball milling; balls need to be made of one of the starting materials (or a similar material) to limit contamination Grinding with salt; salt acts as wedge to break down crystals, after can dissolve the salt leaving only the nanoparticles behind Drawback: can have large differences in size from one particle to the next (bottom-up methods provide much smaller differences in size) 21