Chapter 6 - Examples Flashcards
What are the 5 steps for nanoparticle synthesis?
- mix reactants
- heat/stir
- nucleation
- crystal growth
- precipitation
** nucleation and crystal growth are 2 of the most important
What is the issue with percipitation?
- occurs when NPs are unstable and grow rapidly to precipitate
- want to avoid this when creating NPs
How is precipitation avoided in NP formation
- use active surface agents or bulky molecules
OR - electrostatic stabilization (use charged ions to deal with surface charges of NP (due to defects, adsorption of ions, chain reactions, ect.). The counter ions are adsorbed onto the surface to prevent NPs from agglomerating
What are the 2 structures when producing NPs?
related to energy
- thermodynamic equilibrium limited (low potential E)
- kinetically limited process (high potential E)
*NPs may adopt different structure to bulk
** can turn low E to higher E through thermal annealing
What are properties of MgO?
- highly ionic
- insulating
- non-polar (at bulk)
- high melting temp (3000K)
What are the common lattice structure of MgO?
Rock Salt Structure
- (100) is most stable (thus most common)
- (110) and (111) also occur
What are methods to synthesis MgO?
- PVD
- chemical deposition
- sol gel
What are the 2 structures of MgO in bulk and NP?
bulk (or larger NP) = cubic morphology
NP = hexagonal
bulk vs NP MgO reactivity
bulk = inert
NP = active for oxidation, dehydrogenation, alkylation, and isomerization
Applications of MgO
- insulator
- catalyst
Properties of bulk gold
and structure
- chemically inert
- fcc crystal structure (lattice constant of 4.08A)
- (111) and (100) structure favoured
Synthesis methods for Au NPs
- Plasma sputtering (PVD)
- gold evaporation
- wet synthesis methods
Au NP structure
5 fold symmetry (10-500 atoms)
- N<100: icosahedral (20 sides)
- N<500: decahedra (10 sides)
- N>500: fcc polyhedra
*below that is atomic planar arrangement (<20)
How does strain effects change with Au NP structure?
- 20-sided has increased strain than 10 sides
- strain contributions grow linearly with particle size
- strain is much greater for 20-sided objects and 10-sided
What factors make heterogenous catalysts more reactive?
- decreased size increases exposed surface atoms, increases reactivity
This is explained by:
- confinement of electronic states
- uncoordination of atoms (defects, ect.)
- charges
- NP-support interactions
How does the NP/support interface change reactivity?
- perimeter atoms:
1. may have polarized NP particles which attract the reactants
2. perimeter provides sites for molecules to absorb - non-perimeter
1. NP has defects which could act as active sites
