6. Nanostructured Energy Materials Flashcards
Give a traditional definition of nanotechnology.
The traditional definition of nanotechnology was: “the process of separation, consolidation and deformation of a material by one atom or one molecule.”
What are the changes in the understanding of this technology nowadays?
Today nanotechnology is: “the modification, usage, knowledge and development of nanomaterials, nanotools, nanomachines and nanosystems in order to solve a problem or perform a specific action”.
Explain why nano-materials are so different compared to the bulk “analogs”.
- higher surface area than bulk
- shape (properties depend on their shape), not only on size
This gives: - interesting properties (catalystical and optical properties)
- higher binding energ? mechanical strength
- electrical conductivity
- optical properties
Nanomaterials are so different for several reasons:
a) the smaller the size of the particles become, the fraction of surface atoms compared to bulk atom increases. Larger surface area. Surface sites are generally more reactive.
b) At smaller sizes, the shape become more important. Differences in shape means that the amount of different facets on the particle is different. This is directly related to reactivity. Catalyst application.
c) Quantum confinement effects can also play a role, on for example the optical properties.
Explain methodologies behind the bottom-up and top-down strategies. Analyze pros and cons for their application for energy material design.
Bottom-up: Building up nanomaterials from atoms or molecules. Atoms - clusters - NP!
Examples:
• Colloidal synthesis (metal ions in solution + reducing agent = metal NP, add surfactant to controll size)
• Reverse micelles
Top-down: Refining and reducing bulk materials (bulk - powder - nanoarrangement?)
Examples:
• Mechanical grinding
• Lithography
Explain what the inverse (reverse) micelle method to produce nanoparticles is.
Reverse micelles are structures of surfactants, that have their polar ends in towards the center of the structure and their nonpolar tails out. In these micelles, we can have metal ions that has been solved in water. By adding a reducing agent (e.g. hydrazin, N2H4), we can reduce these ions to their metallic counterparts. By heating we can then dissolve the micelles, and we get precipitation of nanoparticles.
We have good control over the size of these micelles, and thus the nanoparticles, since they are sensitive to the amount of water.
What are common/promising approaches to control the shape of nanoparticles?
-pH (The shape of nanoparticles are often sensitive to the reaction media pH, due to a stronger adsorption of OH- anions at the -facets (so high pH means lots of -facets).)
Bottom-up:
- adding different surfactants
- de-alloying. (Start with an alloy, and then remove the allowed particles. (dissolve) PtNi catalysts. )
- Galvanic displacement (Create nanocages or hollow spheres. Ex: start with a cubic silver NP, then you oxidise the silver when depositing gold, 3 silver is oxidised per gold prodused, will be empty voids inside the gold NP.
Top-down:
- Lithography + templates
-Anodization: Apply some volts: dissolve or oxidose make holes, method to make templates with voids. Ordered voids, tubes. Depends on composition of the electrolyte (F-), and the bias (voltage)
Give typical examples of application of nanostructured materials in current energy science.
Pt3Ni nanoframes are used for oxygen electroreduction, most active catalyst to date. Need a specific shape in order to be best catalyst. Can be made by de-alloying.
Designing electrodes in supercapacitors (need high surface area, C proportional to A)
Usage of Si as intercalation compounds in Li-ion batteries depends on ability to construct optimal nanostructures. Peoply try to use Si instead of graphite. But the drawback is that the dimention of the anode realy expands while charging
Piezoelectric generators (pillars running into each other). To maximice the strains
Nano-structuring of intermetallic compounds for hydrogen storage, can decrease the temperature of desorption of H2.
What are some advantages and disadvantages of ball milling process?
+ very cheap and up-scaleable
- nanoparticle shape and size not uniform
- possible contaminations from the milling balls and attritor walls
- particle size is relatively big (not really nano).
Explain the formation of micelles.
We add surfactants to a solution, where they will form a monolayer on the surface. At one critical micelle concentration, these micelles start to form. The shapes will differ depending on e.g. the packing factor of the surfactant (so we can get cylindrical micelles, lamella structured micelles).
When we add surfactants to a non-polar solvent, we get inverse micelles. These will then have their polar heads in towards the middle, and here water will also fill. The more water we add to the solution, the bigger the micelles will be.
What kind of surfactants exists?
Anionic (negativ polar head), cationic (positive polar head), zwitterionic (two polar groups of each polarity) and nonionic (with polar end groups, but not ionic, e.g. hydroxyl groups).
Why is control of the shape important in heterogenous catalysis?
Because the shape can control the activity of the reaction. Example are nanoparticles of Pt3Ni where the reaction at the oxygen electrode increased.
Increase the reaction in fuel cells
What can be said about the control over reproducibility of the key properties of nanomaterials at a larger scale.
It is not really controllable. Too many things are uncertain, and things can vary from lab to lab.
Why is control of nanostructure important for realizing Si-based intercalation compounds?
The use of Si(anode) in Li-ion batteries, need optimum nanostructures
Because Si has shown to exhibit high expansion when intercalated with Li. The nanostructure must be optimized to limit the amount of expansion. Can achieve a ten-fold increase in capacity.
Give examples of assembeling more complex nanostructures?
- electrophoretic deposition
(have NP then disperse them in a medium with bad conductivity. Then polorise it, apply an electric field, no rxs occure, and the NPs start to go to one of the electrodes. Get packed NPs.
- Langmuir-Blodgett techniques.
(use surfactants, can form monolayers of surfactants (controlled by the surfactant concentration)
