Question 2 Flashcards
q2a)
a) Explain with your own words, what superparamagnetism is. Why is it useful for assembly experiments?
Give one example
Superparamagnetismus: particles are small enough (< 30 nm) to only have one magnetic domain (domain = magnetic moments are aligned)
→ nanoparticle acts like one magnetic moment
→ no hysteresis when changing external magnetic field (ferromagnetic has hysteresis)
Superparamagnetic: no magnetization for H=0 (random oriented) as thermal energy enough to flip magnetization
Ferromagnetic: has magnetization for H=0. Ms = saturation magnetization (all dipoles aligned), Mr: remanence magnetization (magn. in absence of external field), Hc = coercive field (reverse field needed for M=0). Hysteresis obtained bc energy barrier to flip.
Useful: no hysteresis → always the same M for the same H
Example: Responsive photonic nanostructures
→ 100 nm clusters consisting of 30 nm Fe3O4 nanoparticles (superparamagnetic) → outside of cluster: polymer coating for steric repulsion
- when applying external field, nanoparticles & therefore clusters align and come closer together (depends on H, stronger H → particles closer => more blue) → different colors obtained (bc Braggs law)
Why not ferromagnetic: bc there is hysteresis and Mr
How to prove superparamagnetism: show there is no hysteresis
Further: can do this in solids: clusters in resin, apply H for desired color, polymerize resin
q2b)
Why is it important to have a force balance between attractive and repulsive forces in assembly experiments?
Give an example.
Same example as above: responsive photonic nanostructures Synthesis of clusters from Fe3O4 nanoparticles:
- Fe3O4 nanoparticles mixed with oleic acid (red) and DTAB (green) to form bilayer → particle stable in water (head is hydrophilic)
- add Ethylene glycol (EG) to remove some DTAB
→ particles partly hydrophobic, start to cluster - clusters stabilized force balance: when too much EG added: nanoparticles strongly hydrophobic → agglomerate, when not enough EG added → no clustering. force balance between repulsion from DTAB and attraction from hydrophobic part
Clusters:
- Need attractive forces between clusters (magnetic attraction) so they align in lines and get closer with higher magnetic field
- need repulsive forces (polymer / silica coating) so that the clusters don‘t clump together
=> force balance leads to desired distance between clusters (visible light range) to see colors
q2c)
What Is the chemical difference between graphene (G), graphene oxide (GO) and reduced graphene oxide (rGO)?
Graphene (G): single layer of C-atoms in C6 rings (hexagonal lattice), 2D crystalline, fully conjugated (as one electron is free - C bonds to 3 other atoms but has 4 free electrons), thinnest and strongest material, high conductivity, completely transparent, very dense
Top down method to produce rGO (Creation of colloidal suspensions in liquid medium):
1. Oxidation of graphite (hydrophobic) to synthesize graphite oxide (break VdW bonds between sheets, Hummers methods) → graphite oxide is hydrophilic (oxygen groups incorporated at layer
→ polar)
2. Sonificate graphite oxide → exfoliation of graphene oxide (GO) → GO: individual layers, still have O- and OH-groups. In alkaline conditions (high pH) → deprotonate OH-groups → negative charge so there‘s no agglomeration
3. Reduction of graphene oxide to reduced graphene oxide (rGO) → rGO: most O-groups are removed, but not all. Conjugation is not completely back. To prevent agglomeration: use polymers or surfactants.
q2d)
Order these three compounds G, GO and rGO according to their electronic conductivity and explain your answer.
G > rGO > GO
→ G: fully conjugated (pi-orbitals), one free electron per C-atom
→ rGO: partly conjugated
→ GO: not conjugated because electrons used for OH bonds
q2e)
Order these three compounds G, GO and rGO according to their dispersibility in water and explain your answer
G < rGO < GO
→ G: not dispersible bc apolar, is hydrophobic
→ rGO: dispersible due to some remaining OH groups
→ GO: better dispersible because more OH groups
q2f)
Flow reactors exhibit several advantages compared to batch reactions.
List them and explain the differences between the two methodologies
Flow reactor: chemical reactor where reactants are continuously fed into the reactor, and products are continuously removed (important parameter: retention time - time in heating zone).
Batch reactors: reactants are loaded, reacted, and then removed in batches (e.g. cook pasta)
+ More efficient heat transfer & mass transfer
+ short reaction times
+ narrow size distribution
+ larger yield
+ Safety (e.g. with flammable fluid is safe with less fluid)
+ costs (initial costs high, running costs low)
+ on-line monitoring
+ can optimize during the process by changing conditions
limitations: requires immiscible media for droplet formation, reactor expressly designed for the investigated chemical system, reactants have to be soluble in solvent
q2g)
Describe the process of thermally driven crystal segregation to obtain MxOy:SiO2 glass‐ceramics
MxOy:SiO2 glass-ceramic: SiO2 glass with MxOy (ceramic) phases
1. Have MxOy doped SiO2 xerogel (homogeneous & porous)
2. increase temperature → softening
3. increase temperature even more → M switches places with Si to form a more stable phase (nucleation and growth due to higher ionic mobility) & densification
4. MxOy nucleates → biphasic dense glass-ceramic
Number and size of nanocrystals can be tailored by dopant concentration, densification temperature & densification atmosphere (less O2 → more vacancies → easier & faster hopping)
With more dopants (e.g. Sn) → color changes as nanocrystals have different BG
Tuning refractive index with light: with laser deposit energy in nanocrystals → partially dissolve → less nanocrystals => refractive index decreases
q2h)
What is the difference between the citrate synthesis of gold nanoparticles and the Turkevich method?
Citrate synthesis = Turkevich method
but if they mean Brust-Shiffrin (Thiol route), differences:
- citrate route: gold particles are hydrophilic,Thiol route: hydrophobic
- citrate route: negatively charged gold particles in the end, Thiol route: uncharged particles with long alkyl chains at surface
- Thiol route: two-phase synthesis (particles soluble in organic solvent, precursors soluble in water), citrate route: one-phase synthesis
Turkevich Method / Citrate-route
NaAuCl4 in water → AuCl4- + Citrate → Au reduced to Au(0) and this nucleates and forms gold nanoparticles. Citrate on surface
-> Synthesis in water
-> citrate as reducing and stabilizing agent: forms a citrate bilayer, surface is negatively charged (strong pH dependance): hydrophilic
Brust-Shiffrin Method / Thiol route:
HAuCl4 in water, mix with NAuCl4xC in Toluene → 2 phases (Toluene on top), add Thiol (SHxC) and NaBH4 (reducing agent) → Au reduced (stirring needed as Boric acid is in H2O for more interface) and SHxC as surfactants → separate organic phase and wash with EtOH (to remove excess thiol) and cool down (-18°C) → gold precipitates
characterize surface of nanoparticles:
- Qualitatively: FTIR (what it is)
- Quantitatively: TGA (how much is on surface)
Critical thinking:
- thiol is soluble in ethanol bc a bit polar
- Au with thiol is soluble as well, but doesn’t like ethanol as much
q2i)
There are several methods to produce polymeric particles.
What strategies can be used to impart shape or compositional complexity to such particles?
Dispersion polymerization:
monomer soluble in solvent, but polymer is NOT SOLUBLE in solvent, surfactant needed to stabilize particles & inhibit coagulation. Trigger polymerization with initiator → particles swallowed with monomer
→ polymeric particles. Can only have spheres after polymerization, but with heating & stretching can get rice
Mini-/micro-emulsion polymerization:
Reaction medium: mostly water (but 2 phase system, other liquid either insoluble monomer or monomer in insoluble liquid), also need functional monomers (stabilization, crosslinking), initiators, surfactants.
Emulsificate: sonification, stirring, microfluidics (fabricate droplets one by one), maybe can make rice
Microfluidic device:
Droplets of monomers (with photoinitiator and surfactant) in water through a flow reactor, shine light on droplets to start polymerization. Change shape by changing shape of pipe and parameters of droplet generator (polymer concentration <-> particle diameter. Shapes: spheres, cheese, rod
Flow lithography:
→ continuous flow lithography: flowing monomer, shine light in desired shape and polymerize. Different 2D and 3D (with optofluidics) shapes are possible. Can also do with joined two flows
→ stop flow lithography (SFL): monomer flows in, stops, polymerized by light in desired shape, next flow (limited by height of channel and projection shape). Shapes: triangle, ying (from yang), triangle 3d (en Schopf im Bode), brezel, key, pentagon
