Lernmaterialien für Nanotech nanoparticles an der University of Milan

In the bottom-up approach, different exposition could lead to different reactivity-> the reactivity on the surface.

                             PHASE

Nanocubes           (100) 

Nanotetrahedral   (111) 

Cubo-octahedral  (100), (111) 

Nanorod               (110) 

 TM (Tomography)-> used to see the projection of the particles in 2D. The acquisition of images from different positions---> 3D picture--> to recognize the shape

Actractive: 

1) Dispersion (London)

2) Dipol-dipol (Keeson)

3) Dipol-induced dipol (Debye)

Repulsive interactions:

1) Electrostatic

2) Steric

-Rh, Ir, Ru, Os, Pd, Pt carbonylic compounds in the presence of polymer  at high temperature  organosol with particle mean  size 1-10 nm

-Acetate o acyl compounds, Pd(Ac)2, Pd(OAc)2, Pt(OAc)2 in Me,iBut-ketone  at high temperature-> organosol with different size distribution Also applied for bimetallic nanoparticles. 

-Complexes as Ru(COD)(COT), Ni(COD)2, Pt(COD)Cl2 with H2, in the presence of polymers (PVP, cellulose nitrate)-> organosol (d =1-5 nm)

The process is called CVC (chemical vapor condensation). Precursors are vaporized and oxidized in a mixture of propane/oxygen or methane/air  Flame temperature are in the range of 1200-3000K. 

Precursors are different among which metal chlorides are the most popular because of their storage properties and volatility. Problems are connected with acidic vapour and corrosion.  

With this method is also possible to grow surface. When the host surface is flat, alligned growth can occur. The host surface can be also engineered.   

Except Cu and Au, all metals are silver-like when have smooth surface.

When are finely dispersed they are black or brown because of ligh total adsorption due to continuous reflections. Light is converted to heat quantitatively.

When two particles with double electric layer keep rise the repulsion is generated by the overlapping of diffusive double layers.

A minimun of potential energy at short distance determins a stable arrangments of the colloidal particles. Therefore if the potential associated to the double layer is enough high, the electrostatic repulsion will impede the particle to collapse.

LaMere Dinegar model: Nanoparticle formation can be divided in two steps: nucleation and growth. Without stabilizing factors the mormation of massive aggregation is favourite.

Ostwald ripening: phenomenon in liquid sols that describe the changes of structure over time. Over time small sol particles dissolve and redeposit onto larger sol particles.

This is thermodynamically driver and spontaneous. The larger particles are more energetically favoured than smaller particles because smaller particles have more surface than bulk, and surface atoms are energetically less stable than atoms in the bulk. This process predict first the supersaturation of the solution with free atoms, then they condense on the surgace of larger particles.

From precursor molecules/metal atoms to NP through aggregation.

You need to stop the aggregation at the desired size. You can better control the size and the structure respect to top-down approach.

1) Salt reduction (wet methods)

2) Thermal decomposition of organometallics

3) Metal vapor synthesis

4) Colloids stabilized by ligands

5) Micellar synthesis

6) Electrochemical synthesis

COMPONENTS

- Metal salts (they oxidise and are separated with organic solvents or dyalisys or ultrafiltration)

- Reducing agent: hydride, ipofosfite, hydrogen, formaldehyde, tartaric acids, oxalic acid salt

- Stabilizing agent (protective)

The coordination number of the atoms at the surface is smaller than in the bulk and thus the surface atoms result more mobile. 

Since number of surface atoms is growing (decreasing the particle size) the M.P. decreases.