3D: Nanocrystals, nanocomposites, 11-1

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1 Bulk: Films or crystals, amorphous or polycrystaline or single-crystalline 2D: Quantum wells, superlattices, Langmuir-Blodgett films, membranes, plus nanodiscs, nanorolls, nanowalls, 1D: Nanotubes, nanowires, nanorods, nanobelts, 0D: Nano or quantum dots, colloids, nanoparticles Cluster: Objects with up to ~50 units Colloid: Stable liquid phase containing dispersed nanoparticles of nm in size Nanoparticle: Generally nm, with amorphous, aggregates of crystallites or single crystalline 3D: Nanocrystals, nanocomposites, cellular, porous materials, hybrids, Nanocrystal: A single-crystal, nm in polymers. size 11-1

2 Lecture 11 MNS 102: Techniques for Materials and Nano Sciences Module 1: Materials Synthesis Overview Solid-state synthesis; Other methods Strategies for making nanomaterials: Top-down vs bottom-up Bottom-up methods Hydrothermal and Sol-gel syntheses Electrochemical deposition Templates, seed-layers, and catalysts 11-2

3 Materials Synthesis Solid-State Synthesis combines elements and/or compounds without the use of solvents. Raw materials are mixed together, usually as a blend of powders, and the reaction is initiated with heat. In cases where one of the raw materials is volatile, the reaction is conducted under a positive pressure in a sealed container or bomb. After the reaction is complete, the new product with the desired composition is isolated, generally without any washing or other purification steps. Wet-Chemistry Synthesis combines elements and/or complex ions through reaction in solution, as promoted by heat and pressure. The solvent is removed after the reaction, and this will usually be followed by a purification, or washing, step. Any remaining solvent will be removed by a final drying step using heat and/or vacuum to produce the product. Reactive Gas Processing is usually used to produce intermediate and/or final products using reactive gas(es), with appropriate flow, pressure and temperature control. How to MAKE NANOmaterial? 11-3

4 Solid-State Synthesis High temperature direct rxn diffusion limited Steps ( heat & beat or shake & bake ): > Choose precursors > weigh > mix > pelletize > choose container: crucibles/boats ceramic (Al 2 O 3 ~ 1950 C; ZrO 2 /Y 2 O 3 ~2000 C) or precious metals (Ag~960 C; Au~1063 C; Pt~1770 C; Ir~2450 C); or sealed tubes (quartz or SiO 2, Au, Ag, Pt, Nb, Ta, Mo, W) > heat at what T, heating program, in what atmosphere (air, O 2, Ar, N 2, H 2, CO, CO 2, other gas) > grind & analyse; go back to shake & beat if rxn incomplete BUT: could be expensive; rxn incomplete, inhomogeneous products; may not get desired nanostructures 11-4

5 Vapour Condensation & Melt Quenching Vapour Condensation: Thermal decomposition/reaction of precursors in a low pressure flame + rapid cooling of the decomposed products in a cool gas or chilled substrate [e.g. Al 2 O 3, TiO 2, ZrO 2 ] Melt Quenching: Spray plasma over falling powders + melting + rapid cooling in cold water. Source: M. Muhammed, T. Taskalakos, J. Korean Ceramic Soc. 40 (2003)

6 Strategies for making Nanomaterials Source: M. Muhammed, T. Taskalakos, J. Korean Ceramic Soc. 40 (2003) Top-down [Macro-engineering] Mechanical attrition or slicing or ball milling successive cutting of a bulk material to nano size; only mechanical force is used > economical; large scale production possible. BUT: Defects/dislocations; polydispersity; aggregate formation; morphology control difficult Lithographies [Optical, electron-beam, ion-beam] involves etching + deposition + patterning, capable of producing complex materials/systems at will and reproducibly, and for OL cost-effectively. Machining: micro to nanostructures BUT: Expensive; not fast 11-6

7 Bottom-up [Molecular engineering] Vapour-phase, liquid-phase, solid-state reactions, plus mixed phase (L-S) reactions Molecular self-assembly Building blocks + Nano-architectures from building blocks Less defects, more homogeneous, good size and shape control Precipitation/ wet chemical method/ soft chemical method Reduction of metal salt/ solution method Hydrothermal/ solvothermal Thermolysis/ colloidal synthesis Flame synthesis Photochemical synthesis Liquid-liquid interface Synthesis in structural media Sol-gel method 11-7

8 Precipitation/ wet chemical method/ soft chemical method Precipitation see Chem 123 use concept to make new particles & crystals Wet chemistry beaker chemistry or rxns done in liquid phase, e.g. Wet Chemistry Route to Hydrophobic Blue Fluorescent Nanodiamond, Mochalin, Gogotsi, JACS 131 (2009) Soft chemistry Chimie Douce rxns are conducted under moderate conditions (< 500 ); Topotactic = structural elements of reactants are preserved in products but with compositional changes Used to modify electronic structure of solid (doping), design metastable compounds, prepare reactive and/or high-surface area materials Intercalation (ion insertion); de-intercalation; dehydration; ion exchange BUT: Need appropriate precursor; metastable products are unstable 11-8

9 Precipitation/ wet chemical method/ soft chemical method Reduction of metal salt/ solution method Hydrothermal/ solvothermal Thermolysis/ colloidal synthesis Flame synthesis Photochemical synthesis Liquid-liquid interface Synthesis in structural media Sol-gel method Source: Synthesis of silver nanoparticles by chemical reduction method and their antibacterial activity Guzman et al. Int. J. Chem. Biol. Eng. 2:3 (2009) 104. Arrested Precipitation Source: Flame_synthesis_of_carbon_nanotubes.pdf Source: Chemical synthesis of magnetic nanoparticles T Hyeon. Chem. Comm. (2003)

10 Hydrothermal/Solvothermal Synthesis Hydrothermal first used by Sir Roderick Murchison ( ) to describe water action at elevated T and P in causing various rock and mineral formation. Chemical reactions in a sealed heated solution above ambient T and P. Hydro = solvent is water vs solvo = solvent is not water, e.g. GaCl 3 + Li 3 N GaN + 3LiCl in benzene, 280 C Autoclave or Bomb heated above BP in oven. System is always at a non-ideal and non-equilibrium state, while solvent is at its near-critical, critical, or supercritical state. Microporous crystals, superionic conductors, metal oxides, ceramics, zeolites, carbonaceous materials, magnetic materials, phosphers, plus nanoparticles, gels, thin films, helical/chiral structures

11 Advantages Most material can be made soluble in a proper solvent by heating and pressurizing the system close to its critical point; Significant improvement in the chemical activity of the reactant, and in producing materials that cannot be obtained via solid-state reaction; Products of intermediate state, metastable state and specific phase may be easily produced > novel products of metastable state and other specific condensed state; Easy and precise control of the size, shape distribution, crystallinity of the final product through adjusting the parameters such as reaction T, time, solvent type, surfactant type, precursor type; Could produce materials with a low MP, or high VP (that tend to go pyrolysis); Easy, low-cost route to produce new materials Disadvantages Expensive autoclaves; Safety issues during the reaction; Could not monitor and observe the reaction. Difficult to control morphology, size, size distribution Not for all materials Mechanism Usually follows a liquid nucleation model; Different from solid-state reaction mechanism in terms of diffusion of atoms/ions among reactants Enhanced solubility solubility of water increases with T, but alkaline solubility increases much greater with T high ph 11-11

12 Source: Ko et al. Nano Lett. 11 (2011) 666. Nanoforest of Hydrothermally Grown Hierarchical ZnO Nanowires for a High Efficiency Dye-Sensitized Solar Cell 11-12

13 Sol-gel Synthesis Sol-gel process = formation of a network through polycondensation reactions of a molecular precursor in a liquid; excellent for making hard-to-break (high-temperature) material at room or low temperature (with light weight or low density, high porosity/surface area). Sol = a stable dispersion of collodial particles (amorphous or crystalline) or polymers in a solvent [c.f. aerosol same but in a gas]; interact by van der Waals forces or H bonds. Gel = a 3D continuous network that encloses a liquid phase, where the network is formed by agglomeration of colloidal particles (colloidal gel) or particles that contain polymer substructure with aggregates of sub-colloidal particles (polymer gel); covalent interaction > irreversible usually. Steps: Mix colloid to form sol > hydrolysis + condensation > drying to make the desired final forms 11-13

14 Silica Gel Source: Homework 2B: Watch the following 2 videos: In less than 1 page and in point form, identify the strengths and weaknesses of the sol-gel method

15 Electrochemical Cell Design based on Si or ITO Electrode (used for nanoparticle deposition) Outputs the graph A V In a 3-electrode system, the current is passed between the WE and the CE supplied by the reduction reaction, e.g. Cu e - Cu(s) WE WE is kept at constant potential wrt RE. Deposition of metal occurs on the surface of the WE until the surface concentration of metallic ions is depleted. RE CE WE-working electrode [Au/Si or H- Si(100) or ITO electrode] RE-reference electrode (Ag-AgCl electrode) CE-counter electrode (Pt wire) 15

16 Cu Nanocrystals: Diffusion-limited Growth Mechanism 0.2 ma/cm 2 I 2 /I m I (ma/cm 2 ) t (s) UPP OPP I I 2 2 m 0.4 Instantaneous exp t / t 2 m Progressive t / t m nm PPY I I 2 2 m t/t m t / t m 2 1 exp t / t 2 Diffusion-limited instantaneous growth mode effective in the overpotential region Source: Sarkar, Tannous, Zhou, Leung, J. Phys. Chem. B Comm. 107 (2003) m

17 Electrochemical Deposition Used in electroplating technology for making thin films Based on the concept of Reduction-Oxidation rxns at the CAThode and ANode in an appropriate electrolyte an electrochemical cell, i.e. AN OIL-RIG CAT Easy control of size, shape, distribution by applied V, t, electrolyte concentration, ph, conductivity Many scanning modes: Cyclic voltammetry (Current vs Voltage); Potenstiostatic Amperometry Current vs Time at a fixed V; plus many others Simple, flexible, inexpensive to set-up, many variations with both aqueous and non-aqueous electrolytes, used in different sensor and coating technologies BUT: need conductive substrates, e.g. ITO-glass (ITO=Indium Tin Oxide), doped silicon, metals such as gold film, glassy carbon materials need harvesting after deposition; not always uniform/homogeneous 11-17

18 Common Tricks in ALL Syntheses Templates: Well-defined voids in templates (pores, channels, hallow spaces) are used to restrict the growth region in order to guide/develop the desired nanomaterial forms and patterns (nano-molding), e.g. AAO (Anodic Aluminum Oxide) or viruses. Seed layers: Pre-deposited layer used to promote growth of nanostructures in desired morphology, crystalline phases and orientations or on hard-to-deposit substrates; often also used as adhesion layers between two dissimilar materials. Catalysts: Used to promote growth of specific nanostructural materials, with and without orientation/crystallographic alignments, e.g. Au nanoparticles. Note different growth modes: VLS vs VS. Source: Virus Particles as Templates for Materials Synthesis T. Douglas, M. Young. Adv. Mat. 11 (1999)

19 Homework 2C: Read the following review: Template synthesis of nanostructured materials, Y. Liu, J. Goebl, Y. Yin, Chem. Soc. Rev. (2013), In less than 1 page and in point form, identify the strengths and weaknesses of the templating technique