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What is a nanomaterial?
Any matter with at least one of its dimensions in the nanometer scale. Or any matter where at least one of its dimensions is less than 100 nm.
What is a 1-D nanomaterial?
Only one of the nanomaterial’s dimensions is not in the nanometer scale.
What is an example of a 1-D nanomaterial?
Nanorods, nanowires, nanotubes
What is a 2-D nanomaterial?
A nanomaterial where two of its dimensions are not in the nanometer scale.
What is an example of a 2-D nanomaterial?
Nanosheets
What is a 0-D nanomaterial?
A nanomaterial where all of its dimensions are in the nanometer scale.
What is an example of a 0-D nanomaterial?
Nanoparticles, quantum dots
What is the distance of 1 nm measured in hydrogen atoms?
10 hydrogen atoms
What are some of the properties of nanomaterials based on their reduced dimensions
low melting points
Altered lattice constants
Enhanced or new catalytic activity
Stronger interaction with biological systems due to size
New applications such as MEMS, NEMS
What are the two major ways of making nanomaterials?
the top down approach
The bottom up approach
What is the bottom up approach in the synthesis of nanomaterials?
Assemble the material from even smaller components such as molecules or atoms.
What type of methods follow the bottom up approach of the synthesis of nanomaterials?
colloidal synthesis
crystal growth
Surface science
What is the top down approach in the synthesis of nanomaterials?
Takes bulk material and converts it to a nanomaterial.
What type of methods follow the top down approach of the synthesis of nanomaterials?
milling
Etching
Mechanical methods
What ways are there to characterize nanomaterials?
chemical composition
Crystal structure
Morphology
Surface properties
Electronic properties
Optical properties
What are some of the techniques used to characterize nanomaterials?
electron microscopy
X-ray spectroscopy
X-ray diffraction
Electron spectroscopy
What is critical aspect?
It is when a material has a high surface area to volume ratio (nanomaterials have a critical aspect because as things get smaller surface area increases while the volume decreases)
What is different about surface atoms/molecules than inner atoms/molecules in a material?
Surface atoms/molecules have unsatisfied bonds because they are not surrounded by other atoms/molecules like inner atoms/molecules are.
What happens to the bond length between atoms on the surface of a material? And why are the bond lengths different for atoms on the surface than inner atoms?
Because atoms on the surface of the molecule have unsatisfied bonds they experience a net inward force which makes the bond lengths shorter. (Imagine if each atom was a person and each bond was two people holding hands. Inner atoms are like people in the middle of the line, or people who are holding two other peoples hand’s, they are experiencing an equal pull from both people on either side of them so they stay in place and stay an equal distance from each person. Surface atoms are like the people at the end of a line, they only hold onto one persons hand so they are pulled towards that one person, therefore there is a shorter distance between the two people)
What is surface energy?
Surface energy is the potential energy caused by the unsatisfied bonds of surface atoms.
As the size of a material decreases why does the distortion of the surface of the material become more important?
The distortion of the surface of the material becomes more important as the size of the material decreases because the smaller the material is the greater the surface area is compared to the volume of the material.
How do you calculate surface energy?
Where:
γ (gamma) is the surface energy
G is the Gibbs free energy
T is the temperature
P is the pressure
ni is the number of atoms
A is the area of the surface
How do you calculate the surface energy of a new surface?
Where:
γ (gamma) is the surface energy
Nb is the number of bonds broken per atom in creating the new surface
Pa is the number of atoms per unit area of the new surface
E is the bond strength
Do different planes in a crystal lattice have different surface energies?
Yes different planes in a crystal lattice will have different surface energies.
Why is one plane in a crystal lattice preferred over other planes in a crystal lattice?
Because some planes have a lower surface energy materials tend to arrange themselves in a way that will minimize the amount of surface energy in order to become as stable as possible.
What are some methods to reduce surface energy in nanomaterials?
Restructuring a materials surface so that surface atoms can form new bonds to satisfy their bonding structure.
Absorb chemical species on the surface by bonding, electrostatic and van der waals interactions
Crystalline materials can have faceted surfaces to minimize their surface energy
Simple agglomeration. (Nanomaterials will attract each other and clump together to reduce their surface area)
At high temperature the nanomaterials can undergo sintering where they merge together to create a macro structure
Ostwald ripening
What is Ostwald ripening
Larger structures grow at the expense of smaller ones. (This occurs at low temperatures.
What causes Ostwald ripening?
Big crystals have a lower surface energy because they have a more stable center while smaller crystals are unstable and are more likely to dissolve in the water and then attach to the larger crystals.
As the size of a particular decreases what happens to its curvature?
The curvature increases (curvature has an inverse relationship with the radius of a particle)
What is the relationship between chemical potential (U) and curvature of a particle?
Curvature and chemical potential have an inverse relationship. The greater the curvature the smaller the chemical potential
How do you calculate the change of volume of a sphere when atoms are added?
Where:
dV is the change in the volume of the sphere
Ω Is the atomic volume of the added atoms
dn the number of atoms added
dR is the change in the radius of the sphere
How do you calculate the new surface energy of a sphere once atoms have been added?
Where:
µ∞ is the chemical potential of an infinitely flat surface
µS is the chemical potential of the sphere
dn is the number of atoms transferred to the sphere
γ Is the surface energy Of the sphere
dA is the change in the surface area of the sphere due to inclusion of dn atoms
How do you calculate the change in chemical potential of a sphere when new atoms are added? What is this equation called?
Where:
γ Is the surface energy Of the sphere
Ω Is the atomic volume of the added atoms
∆µ is the change in chemical potential
R is the radius of the sphere
This is the Young-Laplace equation
What is the general equation for the Young-Laplace equation?
Where:
γ Is the surface energy Of the sphere
Ω Is the atomic volume of the added atoms
∆µ is the change in chemical potential
R1 is one of the principle radii of the surface
R2 is another of the principle radii of the surface
What does the Gibb’s Thompson equation calculate?
It relates how the curvature of an object affects its solubility.
What is the Gibb’s Thompson equation?
Where:
γ Is the surface energy Of the sphere
Ω Is the atomic volume of the added atoms
R1 is one of the principle radii of the surface
R2 is another of the principle radii of the surface
Sc is the solubility of the surface where R1 and R2 as the two principle radii
S∞ is the solubility of an infinitely flat surface of the same material.
What does the Gibb’s Thompson’s equation tell us?
The equation tells us that smaller particles have a greater solubility
What are the two primary methods of stabilization of nanomaterials?
Electrostatic stabilization
Steric stabilization
Why do we have to stabilize nanoparticles?
If we do not stabilize nanoparticles they will agglomerate to reduce their surface area and therefore reduce their surface energy. Once they have aggregated it is super hard to get them back to how they were before
Why are nanomaterials commonly synthesized in an aqueous medium?
the solvent is nontoxic
The solvent can be used to further process the nanomaterial
Why do solid surfaces in a polar solvent develop a surface charge?
dissociation of surface species on the material (certain ions will dissociate in water which will cause the surface to become charged)
Absorption of charged species onto the surface such as ions or molecules. (So if there was something like chloride ions in the solvent they might get absorbed by the nanomaterial which would cause the surface to become negatively charged)
Accumulation/ depletion of the electrons on the surface. (when the surface ions have unsatisfied bonds they will undergo reduction/ oxidation)
What will happen if a charged nanomaterial is in an aqueous medium?
The nanomaterial will attract oppositely charged ions and repel similarly charged ions.
What causes the redistribution of ions when a charged nanomaterial is in an aqueous medium?
Columbic/ electrostatic interactions (like charges repel opposite charges attract)
Thermal energy ( increasing the energy of a system allows ions to overcome the attractive force and essentially make the ions less attracted to each other)
Entropic forces (look this up later)
What is the electrical double layer?
When there is a charged surface in a polar solvent, ions will distribute into a double layer structure on the surface.
How the the electrical double layer different from the bulk solution
In the bulk solution there is a random and equal distribution of all the ions but in the electrical double layer the ions are concentrated.
What are the two primary layers in the ion distribution in the electrical double layer?
the stern layer
The diffuse layer
What is the stern layer in the EDL?
It is the first layer on the charged surface. It is made up of ions with the opposite charge of the charged surface (if the charged surface has a positive charge then the stern layer is made up of ions with negative charges). These ions are very strongly bonded to the surface which means they are not free to move away from the charged surface.
What is the diffuse layer in the EDL?
The diffuse layer is the second layer in the EDL. These ions are not as strongly bonded to the charged surface and so are free to move around.
What happens to the electrical potential across the stern layer?
The electrical potential drops linearly
What happens to the electrical potential across the diffuse layer?
The electric potential drops exponentially.
What is the Debye length?
It provides a measure of the length scale of the diffuse layer. (It’s basically the length of the diffuse layer kinda)
What is the Debye length dependent on?
It is dependent on the concentration and the valence of the ions in the solution.so if the concentration of the ions increases then the dyne length will decrease and repulsive energy will decrease.
How does electrostatic stabilization work?
It is dependent on the concentration and the valence of the ions in the solution.so if the concentration of the ions increases then the dyne length will decrease and repulsive energy will decrease.
What are the two forces acting on electrostaticly stabilized particles?
Basically because the surfaces of the nanoparticles are charged with the same charge the participles repel each other making the solution stable.
What causes van der waals forces?
The are caused from the motion of electrons in an atom and the resulting instantaneous dipoles (polarization of the atom)
Why are van der waals forces important in nanoparticles?
The sum total of all the atoms and molecules in the nanoparticle create a Van der Waals interaction energy for the nanoparticle.
What will happen to the repulsive force between two nanoparticles in an aqueous solvent if the concentration of salt increases in the solvent?
The repulsive force will decrease and the nano materials will no longer repel each other.
Do you want the thermal energy of the solution to be greater than or less than the repulsive energy between two nanoparticles?
You want the thermal energy to be less than the repulsive force. If the thermal energy is greater then the particles will be moving fast enough to overcome the repulsive force and the nano particles will stick together.
What is the van der waals energy proportional when two nanoparticles are near each other in a polar solvent.
The van der waals energy between to particles is proportional to the distance between the two particles.
What are the assumptions of the DLVO theory?
Uniform charge density on the particles
As the particles get closer the counter ions do not redistribute (EDL does not change)
Solvent is only a dielectric medium (it does not interact with the electric field caused by the charged particles)
At the primary maxima in the DLVO theory what is the behaviour of the particles like?
At the primary maxima this is an energy barrier. The particles repel each other because they want to reach the lowest energy possible. They would need a lot of energy to overcome the primary maxima. If they did manage to over come it then the particles would agglomerate. If two particles did not have a primary maxima then all the particles would agglomerate.
The the secondary minima for the DLVO theory what is the behaviour of the particles?
The particles maintain a stable distance from each other because the energy is at its lowest. It would take energy for the particles to get closer and it would also take energy for the particles to separate any further.
Which criteria is required for the DLVO theory to be applicable?
dilute solution of nanoparticles (there cannot be a lot of nanoparticles)
Simple geometry of nanoparticles
Presence of EDL
Only interactions between particles are bad der waals interactions and electrostatics.
What is steric stabilization?
It is a way to stabilize nanoparticles by interfacing/ bonding of polymer chains to the surface of nanomaterials.
What principles does steric stabilization use?
It uses thermodynamics.
What is the benefit of steric stabilization?
If the nanoparticles agglomerate they can be re-dispersed.
Also particles can be suspended in high concentrations and it does no affect the stability.
What are the two ways that polymer chains can be attached to a nanoparticles?
irreversible bonding
Absorption of the chains on the surface of the particle.
What does irreversible bonding of a polymer chain to a nano particle look like?
It kind of looks like egg fertilization
What does it looking like when polymer chains are absorbed by the nanoparticle?
Multiple points of a polymer chain touch the surface of the nanoparticle
What makes a good solvent for steric stabilization?
If the polymer chains expand in the solvent to increase its interaction with the solvent molecules then, it is a good solvent.
Why is a good solvent for steric stabilization one which utilizes a solvent that encourages the polymer chains to expand in the solvent?
You want the polymer chains to expand in the solvent because it means that solvent-polymer interactions are more energetically favourable (Gibbs energy) than polymer-polymer interactions. Basically the polymer chains from one nanoparticle repel polymer chains from other nanoparticles which keeps them from agglomerating (which is what we want).
What happens when steric stabilization uses a poor solvent?
The polymer chains collapse in on themselves because in a poor solvent the gibbs free energy is reduced the most between polymer-polymer interactions. We dont like poor solvents because it means that the polymers of one nanoparticle will attract the polymers of another nanoparticle causing agglomeration.
What is a factor that affects polymer solvent interactions in steric stabilization?
Polymer-solvent interactions are temperature dependent.
For a good solvent what is the temperature at which there is not difference between polymer-polymer and polymer-solvent interactions called?
It is called the flory-Huggins temperature
What happens To Gibbs free energy at the Flory-Huggins temperature?
The Gibbs free energy does not change irrespective of whether the chain collapses or expands. (Gibbs free energy is not dependent on what the polymer chains are doing)
For a good solvent what happens when the polymer chain coverage of the nanoparticle is less than 50% and the distance between the particles is greater than double the length of the polymer layer of the nanoparticles?
The polymer chains of the nano particles do not interact and therefore there are only polymer-solvent interactions.
For a good solvent what happens when the polymer chain coverage of the nanoparticle is less than 50% and the distance between the particles is between one layer of the polymer chains and two layers of the polymer chains?
There will be polymer-polymer interactions and polymer solvent interactions.
For a good solvent what happens when the polymer chain coverage of the nanoparticle is less than 50% and the distance between the particles is less than one layer of polymer chains?
The polymer chains on one surface will also interact with the surface of the other nanoparticle.
For a good solvent what happens to Gibb’s free energy as two nanoparticles approach each other?
There will be an increase in the polymer-polymer interactions, however since we know that in a good solvent, polymer-solvent interactions are more energetically favourable Gibb’s free energy will increase. When the polymer chains begin to interact with each other their entropy will be reduced (because their ability to move becomes restricted by other polymer chains). using the formula ΔG = ΔH - TS we can see that as the entropy decreases that Gibb’s free energy will increase since the enthalpy will not have any noticeable changes. The goal of every system is to decrease its Gibb’s free energy and so to do this the polymer chains will repel each other to increase their entropy again.
In a good solvent what happens if the polymer chain coverage on a nanoparticle is equivalent to 100%?
When this happens, if two nanoparticles approach each other their polymer chains will coil up like springs and push each other away because again the particles prefer polymer-solvent interactions over polymer-polymer interactions.
When a poor solvent is used what happens when the polymer coverage of a nanoparticle is low?
The nanoparticles are attracted to each other and there for will try to agglomerate.
When a poor solvent is used what happens to the nanoparticles when the coverage is low but the distance between the particles are less than the thickness of the polymer chains.
The chains will compress which will decrease the entropy (because the chains will not be able to move freely) which will cause the polymer chains to repel eachother. (Basically when the solvent is bad the particles will agglomerate and find an equilibrium position that is a certain distance away from other particles because the polymer chains are attracted to each other and want to interact but they do not want to be compressed)
For a poor solvent what happens when the polymer chain coverage of the nanoparticle is super high?
The polymer chains will coil up and repel each other because their entropy will increase (because they are not able to move as much) and therefore will repel each other even though polymer-polymer interactions are energetically favourable.
For the bottom up approach of nanoparticle synthesis what are the two different types of nucleation?
There is homogenous and heterogeneous nucleation.
What principle governs colloidal synthesis?
Thermodynamics and kinetics govern colloidal synthesis
What are some challenges of top down synthesis?
size
Creating uniform particles
Getting evenly shaped particles
Composition
Prone to impurities
It can take a long time to form particles in this manner.
What are the two principles that govern colloidal synthesis?
Thermodynamics
supersaturation
Nucleation
Growth
Kinetics
rate of formation and growth
Can be used to limit the size of the particles to the nanometer scale.
What does it mean when a solution is super saturated?
When a solution is super saturated it means that there is more solvent in the solute than is energetically favourable.
What happens to Gibb’s free energy when a solution is super saturated?
The Gibb’s free energy is very high. In order to reduce the Gibbs free energy some of the solute will change phases which will reduce the concentration. (Think of sugar in water. If you keep adding sugar to water at some point you won’t be able to add anymore and sugar will begin to form at the bottom of the glass).
What is the concentration at which no more solvent can be added, called?
It is called the saturation concentration
If the concentration of the solution is equal to the saturation concentration what happens to Gibb’s free energy?
This point is when the solution is the most energetically stable, therefore Gibb’s free energy is equal to zero.
When the concentration of the solution is greater than the saturation concentration what happens to Gibb’s free energy and the solution?
At this concentration Gibb’s free energy will be greater than zero. To reduce Gibb’s free Energy the solvent will begin to form a solid in order to get back to the saturation concentration and make Gibb’s free energy zero. Therefore Gibbs free energy will be reduced. However Gibbs free energy will also increase. Since a solid is forming this solid will also have a surface energy which will increase gibb’s free energy
What happens when the concentration of the solution is less than the saturation concentration?
Gibb’s free energy is greater than zero however no precipitate is formed because the concentration is not higher than the saturation concentration.
What does this formula tell us about a supersaturated solution?
It tells us that the change in Gibbs free energy will be at a maximum when the radius of the particles that form as a precipitate are of a particular size. If the radius of these particles are greater than this radius then the particles will continue to grow until they reach an equilibrium radius where gibb’s free energy is equal to zero. If the radius of these particles is less than this radius then they will re-dissolve to reduce gibb’s free energy.
For a supersaturated solution how do you reduce the size of the radius at which Gibb’s free energy is the highest?
Reduce the degree of supersaturation.
Reduce the surface energy of the new phase (how would you do this)
What does the rate of nucleation depend on?
the probability to overcome the barrier for nucleation (when gibb’s free energy is the highest because the radius is at that weird point)
The number of growth species per unit volume (the concentration)
The diffusion of the growth species (the diffusion velocity)
What factors will increase the rate of nucleation?
a low solution viscosity
A high temperature
A high concentration of growth species
A low species size
A low Gibb’s free energy for the max Gibb’s free energy number
While increasing the concentration of the solution at what point on this pragmatic will there be nucleation and growth of particles?
Growth and nucleation will occur between Cmin and Cmax
What is Cmin?
Cmin is the concentration at which nucleation and growth will occur in order to reduce the concentration of the solution. Growth will also occur as long as the concentration is above the equilibrium concentration.
What are the two steps involved in the growth of nuclei?
Supply the growth species to the surface of the particle nuclei. (Diffusion + absorption)
Particle grows by incorporating the growth species into solid phase (surface process)
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