Nanoparticle Synthesis and Emulsions

Question 1

Explain the difference between the two main approaches (hot injection and heat-up) within colloidal synthesis that are used to obtain uniform nanoparticle size?

Answer 1

• Hot-injection: One reagent is added quickly to boost the supersaturation (cold in hot)
• Heat-up: All reactants are added prior to heat-up. Supersaturation reaction during heating of all reactants

Question 2

Explain LaMer theory using the terms monomer accumulation, homogeneous nucleation, diffusion controlled growth, critical supersaturation.

Answer 2

(Figure/graph)
Stage 1: Production and accumulation of monomers
Stage 2: Concentration cross supersaturated solution, reach critical concentration needed to nucleate homogeneously
Stage 3: Growth phase - remaining monomers are used for growth. Size focusing will take place here.

Question 3

What is meant by the term “diffusion controlled growth”?

Answer 3

Transport of monomer to the surface is limiting the growth (Small, 2011)

Question 4

What is meant by size focusing”, and in which part of the LaMer diagram will this process take place?

Answer 4

The growth rate will decline with increasing radii of the particle (smaller particle, faster growth). When the particle gets bigger, the growth rate decrease.
The process takes place in stage 3, growth controlled diffusion.

Question 5

Explain the concepts “Ostwald ripening”, “coalescence” and “oriented attachment”. Are these phenomena described in the LaMer theory? Justify your answer

Answer 5

• Ostwald: Smaller particles are less stable, goes into solution, and the particles/monomer deposit on larger particles
• Coalescence: When two or more particles fuse into one
• Oriented attachment: Certain facets are more energetically favorable

Question 6

Explain in terms of thermodynamics at which conditions a nuclei become stable and continue to develop into a particle.

Answer 6

Important: Surface energy become essential and hinder nuclei stabilization when particle is small.
(explain equations when presented, plus graph)

Question 7

Differentiate between homogeneous and heterogeneous nucleation, and explain which nucleation mechanism that is less energy demanding.

Answer 7

• Homogeneous: Sponaneous nucleation from main matrix due to supersat.
• Heterogeneous: Nuclei form on already existing surface. Less energy demanding.
  Contact angle small –> good wetting, cos(contact angle) goes to 1 –> less energy demaning (Gibbs)

Question 8

When solidifying a melt in a mold, normally where in the mold will you expect the nucleation to be initiated and in which part of the mold will solidify last?

Answer 8

Solidification starts at wall (heterogeneous)

Question 9

Explain why it may be important to control particle size, particle morphology, chemical composition-element distribution as well as contamination levels in nanoparticles synthesized for applications

Answer 9

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Question 10

Explain the principles behind CdSe synthesis (include the term Ksp in your explanation).
Explain how CdSe particle size control their optic properties.

Answer 10

CdSe (hot injection)

Question 11

What is the plasmon effect?

Which physical parameters of the nanoparticle is critical to control in order to tune the plasmon effect?

Answer 11

Electrons mean free path is 5-60 nm. When particle enter this size range, electrons are scattered at the inner surface giving rise to surface plasmons At a specific wavelength (freq) of light, collective oscillation of electrons on e.g. a gold NP

Question 12

Explain the principles behind cobalt nanoparticles synthesis. In your discussion, touch upon polymorphism for Co (which atomic arrangements are known) and possible size induced changes in magnetic properties when going down in size for a ccp Co particle).

Answer 12

Burst nucleation: formation of large number of nuclei in a short period of time, then growth without additional nucleation.

hcp: stable bulk below 420 C
ccp: stabilized as NPs
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Question 13

Explain the phase diagram for Pt-Rh. In synthesis of Pt-Rh NPs suggest an explanation to that both core-shell and solid solution Pt-Rh NPs can be obtained.
Be ready to justify for various element distributions in bimetallic nanoparticles; synthesis parameters that can be mofigied to tune element distribution in the NPs.

Answer 13

Thermodynamics - solid solution despite de-mixing indicated by phase diagram. Reaction kinetics of Pt and Rh are key parameters, as one might be faster than the other, which can lead to a core-shell structure. etc

Question 14

Normally we use a heating element/block to heat up a solution during nanoparticle synthesis. Microwave assisted nanoparticle synthesis is also an approach frequently used to obtain nanoparticles. Explain the physical principles behind this approach.

Answer 14

Microwaves: Electromagnetic waves that induce alternating polarization that again creates vibration = heat.
Based on a materials ability to absorb microwave enegry and convert it into heat
(Dipolar polarization + ionic conduction)

Question 15

Which three types of emulsion do we discuss in this course?

Answer 15

• Macro: Not too interesting in this course (?)
• Micro: Clear, thermod. stable, isotropic liq. mix of O+W+surf. Spontaneous formation
• Nano: Clear, kin. stable liq. mix of O+W+surf. Not spontaneous.

Question 16

Which three constituents are required to obtain an emulsion? How will the ratio between these constituents define the nature of the emulsion?

Answer 16

1) Oil-phase
2) Water-phase
3) Surfactant
3-way phase diagram
(oil-in-water, water-in-oil)
Nanoreactor: Confined medium where a nanoparticle synthesis can be performed (reverse micelle)

Question 17

Suggest an approach that would allow you to produce monodisperse PtRh alloyed nanoparticles in reverse micelles. Justify for your choice of reactant.

Answer 17

W: Rh(NO3)3 + H2PtCl6
O: n-heptane
Surf: polyethylene glycol dodecyl ether