Question 1 Flashcards
q1a)
Why is it important to have directional forces in an assembly experiment?
Give an example with and without directional forces.
Directional forces important for assembly of structures (e.g. with hierarchy), can be very precise, you get non-close-packed and anisotropic structures. Possible to control the directionality (in 1D)
With directional forces (oriented attachment): oriented attachment of surface-functionalized TiO2 electrostatically stabilized by Trizma
- Synthesize surface-functionalized TiO2:TiCl4 with benzyl alcohol → TiO2 and stabilize with Trizma (the 3 OH bind to the TiO2 and the N is positively charged: red in illustration)
- selective removal of Trizma from the {001} crystal facets by water → you get the blue parts (=destabilized part)
3: condensation of water/-OH groups → you get wires (3D structure possible if not enough Trizma → branching)
from 0D to 1D (to 3D: use less Trizma)
Without directional forces (Attractive and repulsive forces without directionality):
Agglomeration of particles, agglomerates when there is a net attraction (e.g. van der Waals) and an equilibrium separation (attractive & repulsive forces), close-packed & isotropic structures
e.g.: agglomeration of hydrophobic particles in water
q1b)
What are the Peclet number and the Reynolds number?
Why is fluid mechanics important to design a flow reactor?
Pe = advection / diffusion (dimensionless)
→ how heat is transported in the flow. Advection or diffusion dominated, Pe < 1: diffusion dominated
Re = rhodv/mu (dimensionless)
→ rho = diffusion; d = relevant dimension e.g. width, depth, diameter,…; v = velocity; mu = viscosity how mass is transported by the flow? Turbulent (Re»_space; 1) or laminar (Re < 1)?
Most important for type of flow: viscosity & density
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)
Important for:
* efficient mixing (laminar flow: no mixing, important for segmented flow: droplets need to mix inside)
* uniform properties (laminar flow: retention time not the same if in middle or at wall)
* Heat transfer: when conventional heating: important to know if diffusion controlled (→ dimension of pipe or not)
* better control of size distribution, reaction time with flow rate
Important to know dimensions of reactor (geometry, distance = retention time), type of flow (continuous, segmented: in the droplets convective mixing and no contact to the wall → when heating up: overcome deposition at the walls of tubing) and operating conditions (state of fluid, pressure)
conventional heating vs microwave heating: conventional heating not efficient at high flow rates and nanoparticles grow at walls of tubing. segmented flow: droplets have no contact to wall
q1c)
What are the advantages and limitations imposed by the use of solvents in the supercritical state during the flow synthesis of NPs?
Supercritical fluid: properties of fluid can be tuned from liquid-like to gas-like
Pros:
+ extremely mobile
+ has density of liquid (a lot of molecules)
+ high miscibility and fast diffusion rates = fast reaction time
+ can dissolve a wide range of compounds
Cons:
- expensive
- high pressure & temperature
- Need to be precise
- Reactors made out of metal → on-line monitoring only possible with high energy radiation (e.g. synchrotron radiation)
q1d)
What is meant by the term “size focusing” in the context of nanoparticle synthesis? Why is it important?
Size Focusing: The smaller particles grow faster than the larger ones. Bc its diffusion controlled growth (growth rate follows 1/r) → after some time every particle the same size. Diffusion controlled: low surface energy, high supersaturation
Important to have a more narrow size distribution → uniform properties (e.g quantum dots, important to have same optical and electronic properties)
q1e)
The picture below shows the fluorescence spectra of quantum dots with different sizes.
Draw, just qualitatively, the relative diameters of the quantum dots above the spectra.
Give a rough estimation of the size of the smallest and largest quantum dots.
Fluorescence: when electron drops from CB to VB
The smaller the QD, the larger the BG → higher energy wave (smaller wavelength) needed to excite electron → blue is emitted
Particle size range: 2.5 - 6.3 nm (for CdSe, in general: 2-10nm, below Bohr radius)
q1f)
Sketch qualitatively the band structure of the smallest and the largest quantum dots.
Label the bands and explain, based on your sketch, the different colors.
smallest QD: larger energy gap between CB and VB bc quantum confinement (exciton is closer together, energetically less favorable)
=> needs higher energy wave to excite electron from VB to CB
→ emitted light (fluorescent, shine UV light): you see the color corresponding to the energy difference
→ In white light: this color absorbed
q1g)
What is the reason for such a quantum size effect?
Explain briefly in your own words
Quantum size effect: Typically for particles below 10 nm (100-10k atoms), size in transition regime between bulk material a molecule, chemical and physical properties become size dependent (can be different from bulk) => we can change properties by changing the size, size control is important for uniform properties.
Reason for QSE:
- exciton (electron-hole-pair) is spatially confined (no bands but individual energy levels): bc electron and hole cannot achieve desired distance (Bohr radius) in nanocrystal. Max size of nanocrystal depends on Bohr radius
- Nanocrystal has additional energy (compared to bulk) due to quantum confinement (Brus equation)
- The smaller the box the larger the energy (Brus equation)
Surface plasmons: oscillation of electrons in conduction band at nanoparticle surface upon excitation with light
q1h)
Please give a definition of “colloidosome”.
Explain briefly, how “pickering emulsions” can be treated in order to form a colloidosome.
Colloidosomes: Selectively permeable capsules composed of colloidal particles, object made of colloids. (colloid = mixture of small particles that stay distributed), can be used for drug delivery
→ properties: complex, smart materials, selectively permeable, shells consisting of particles, responsive to different triggers (pH, temperature, magnetic field,…)
→ Influence permeability: size of particles, sintering time (how much particle are sintered together), particles of different shapes
Pickering emulsion: particle stabilized emulsion
Steps:
1. Mixture of water and oil and particles (particles have similar affinity to both phases, amphiphilic)
2. mix → particle stabilized emulsion: interface is stabilized by adsorption of particle, inner phase and surrounding phase are immiscible
3. Lock the particles at the interface: sintering, polyelectrolyte (electrostatic binding: charged polymer chains go around the colloidosome), removing steric hindrance, polymerization of droplet phase/gel trapping, crosslink ligands on particles, crosslinking of polymer spheres (when colloids are polymers)
q1i)
Taking the hot injection method as an example, explain the nucleation and growth of nanoparticles according to the LaMer diagram.
Plot separately saturation, concentration of nuclei, size, and size distribution of nanoparticles as a function of time.
Hot injection mode:
1. injection of a cold precursor (organometallic of metal organic compounds/metal salts) solution to a hot surfactant (polar head group and long alkyl chain) or surfactant/solvent mix
2. nucleation and growth
3. Surfactant controls size and surface properties
4. precipitation by addition of a “nonsolvent
Steps:
- Monomer accumulation: precursor starts to react into monomer, rapid accumulation
- Homogeneous Nucleation: when high enough supersaturated (S = [Monomer]actual/[monomer]eq). S»1 (needs to be high for smaller energy barrier and smaller critical radius for nucleation) => nucleation (monomers consumed to form particles)
- Diffusion controlled growth: growth rate proportional to 1/r -> size focusing. Surfactants can increase or decrease crystal size (depends on where the surfactant attaches: precursor or monomer). Mean particle radius: first fast growth and then slower. Particle size distribution gets narrower due to size focusing.
=> for monodisperse crystal: separation of nucleation and growth (nucleation rate ideally zero during growth as don’t want new (and small) particles)
concentration of nuclei:
- as we want small size distribution: the nuclei should nucleate/grow at the same time → we want “vertical” line
If we wait too long/less monomers available → Ostwald ripening (size defocusing)
q1j)
What is the difference between aqueous and nonaqueous sol‐gel chemistry?
→ both methods can produce monodisperse metal oxide particles
aqueous: Molecular precursor + water → inorganic polymerization reactions (Hydrolysis and condensation= => metal oxide network)
e.g. Stöber particles (hydrolysis of tetraethoxysilane, needs very high or very low pH for fast reaction) nonaqueous: „molecular“ precursor + organic solvent => metal oxide network
difference:
- aqueous: usually at RT and very quick (except: Si relatively slow), eco-friendly, but less control over reactions (bc fast), limited to water-stable precursors
- nonaqueous: slow → heating, no water involved (e.g. alcohols for condensation), better control over reaction, more complex and expensive
