• 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

• 3D: Nanocrystals, nanocomposites, cellular, porous materials, hybrids, polymers.

11- 1

• Cluster: Objects with up to ~50 units

• Colloid: Stable liquid phase containing dispersed nanoparticles of 1-1000 nm in size

• Nanoparticle: Generally 1-100 nm, with amorphous, aggregates of crystallites or single crystalline

• Nanocrystal: A single-crystal, nm in size

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

2 11-

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.

11- 3 How to “MAKE” NANOmaterial?

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 (Al2O3~ 1950C; ZrO2/Y2O3~2000C) or precious metals (Ag~960C; Au~1063C; Pt~1770C; Ir~2450C); or sealed tubes (quartz or SiO2 , Au, Ag, Pt, Nb, Ta, Mo, W) > heat at what T, heating program, in what atmosphere (air, O2, Ar, N2, H2, CO, CO2, 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

Vapour Condensation & Melt Quenching

11- 5 Source: M. Muhammed, T. Taskalakos, J. Korean Ceramic Soc. 40 (2003) 1027.

• 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. Al2O3, TiO2, ZrO2]

• Melt Quenching: Spray plasma over falling powders + melting + rapid cooling in cold water.

Strategies for making Nanomaterials

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 Source: M. Muhammed, T. Taskalakos, J. Korean Ceramic Soc. 40 (2003) 1027.

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

11- 7

• 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

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) 4594 http://pubs.acs.org/doi/pdf/10.1021/ja9004514

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

11- 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.

Source: “Chemical synthesis of magnetic nanoparticles” T Hyeon. Chem. Comm. (2003) 927.

Arrested Precipitation

Source: http://cdn.intechopen.com/pdfs/16801/InTech-Flame_synthesis_of_carbon_nanotubes.pdf

Hydrothermal/Solvothermal Synthesis

11- 10

• “Hydrothermal” first used by Sir Roderick Murchison (1792-1871) 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. GaCl3 + Li3N → 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- 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- 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”

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 sub-structure with aggregates of sub-colloidal particles (polymer gel); covalent interaction > irreversible usually.

11- 13

• Steps: Mix colloid to form sol > hydrolysis + condensation > drying to make the desired final forms

Silica Gel

11- 14

Source: https://www.llnl.gov/str/May05/Satcher.html

Homework 2B: Watch the following 2 videos: http://www.youtube.com/watch?v=VlWGIKCV_6k

http://www.youtube.com/watch?v=35IgXnXnA1Y

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

CE RE

WE

• WE-working electrode [Au/Si or H-Si(100) or ITO electrode]

• RE-reference electrode (Ag-AgCl electrode)

• CE-counter electrode (Pt wire)

• In a 3-electrode system, the current is passed between the WE and the CE supplied by the reduction reaction, e.g. Cu2+ + 2e- Cu(s)

• 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.

16

Cu Nanocrystals: Diffusion-limited Growth Mechanism

1 2 3 4 5 6 70.0

0.2

0.4

0.6

0.8

1.0

Instantaneous

Progressive

I2/I

m

2

t/tm

0 2 4 6 8 10

0.4

0.8

1.2

1.6

2.0

Sarkar etal. Fig. 2/3

I (m

A/c

m2)

t (s)

2

2

2

/2564.1exp1/

9542.1m

mm

ttttI

I

22

2

2

/3367.2exp1/

2254.1m

mm

ttttI

I

Diffusion-limited instantaneous growth mode effective in the overpotential region

Source: Sarkar, Tannous, Zhou, Leung, J. Phys. Chem. B Comm. 107 (2003) 2879.

0.2 mA/cm2

100 nm PPY

UPP OPP

Electrochemical Deposition

11- 17

• 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

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.

11- 18

Source: “Virus Particles as Templates for Materials Synthesis” T. Douglas, M. Young. Adv. Mat. 11 (1999) 679.

11- 19

Homework 2C: Read the following review:

“Template synthesis of nanostructured materials”, Y. Liu, J. Goebl, Y. Yin, Chem. Soc. Rev. (2013), http://pubs.rsc.org/en/content/articlelanding/2013/CS/C2CS35369E. In less than 1 page and in point form, identify the strengths and weaknesses of the templating technique.