Chapter 6 - Nanostructured energy materials Flashcards
Give a traditional definition of nanotechnology. What are the changes in the understanding of this technology nowadays? Explain why nanomaterials are so different compared to the bulk analogues.
The traditional definition of nanotechnology was: “the process of separation, consolidation and deformation of a material by one atom or one molecule.”
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”.
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. 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.
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: means that we produce the material from the bottom, by e.g. growing it using molecular beam epitaxy or synthesizing nanoparticles using reverse micelles. This is not always easily scaled up.
Top-down: we start with a bulk material, and modify it to achieve the nanostructures we want. This can often be more easily implemented on large scale facilities, such as the lithographic process of making nanostructures on wafers.
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?
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).
We can also control shape by dealloying. Start with an alloy, and then remove the allowed particles.
We can also use anodization, where we use a nanostructured template, and apply a voltage to drive a reduction reaction to fill the pores.
During vapor deposition, we can adjust some of the reaction parameter to change the growth rate in the different directions.
Give typical examples of application of nanostructured materials in current energy science.
Pt3Ni nanoframes are used for oxygen electroreduction, most active catalyst to date.
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.
Piezoelectric generators (pillars running into each other).
Nano-structuring of intermetallic compounds for hydrogen storage.
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).
How can carbon nanotubes be grown in a CVD-chamber?
By using CxHy (eg. CH4) percursor gas, and pass it through a chamber with iron droplets deposited. Then through VLS mechanism.
What is anodization?
Find out.
How does cathodic corrosion work? Why is this counter-intuitive?
Find out.
Why is control of the shape important in heterogenous catalysis?
Because the shape can control the activity of the reaction. Example are nanoparticles of TiO2 where the H2 production rate was much higher for a special shape of the nanoparticle.
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?
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.
