Page 57 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 1 BASIC PROPERTIES AND MEASURING METHODS OF NANOPARTICLES
Table 1.9.1
Physical surface relaxation.
Crystal structure (110) (001) (111)
Cu Face-centered cubic 0.804 0.871 0.944
Al Face-centered cubic 0.900 1.00 1.15
Fe Body-centered cubic 1.00 0.986 0.85
NaF Face-centered cubic – 0.972 –
NaCl Face-centered cubic – 0.970 –
NaBr Face-centered cubic – 0.972 –
MgO Face-centered cubic – 0.85 –
Note: Values in this table indicate lattice gap between the 1st and 2nd layer divided by lattice gap for bulk crystal.

faces and bonding energy. Similar relaxation occurs On the surface of nitrides and fluorides, the chemical
on surface of metal crystals, and the surface relax- adsorption of water vapor takes place and then oxides
ation is stronger on a crystal face having a lower are formed as shown in the following equations.
atomic density. In this case, the region in which the Eventually, the surface of the oxides is hydrolyzed due
relaxation occurs would be up to around two layers. to further chemical adsorption of water vapor.
It has been already described that solid surfaces are
highly reactive since they are unsaturated in terms of Si N
2 O 3SiO 4NH ,
6
3
4
3
2
bonding. It is known for a solid left in the atmosphere Si- -Si
2 Si-OH (1.9.5)

that the highly reactive surfaces react chemically with 2
gases in the atmosphere such as oxygen, carbon
6
dioxide, water vapor, and then form a surface compo- 2TiB
2 O 2TiO 2B H O , (1.9.6)
2
2
6
2
2

sition different from chemical composition of the bulk. Ti- -Ti
2 Tii-OH
2
Oxidization and erosion of metal surface due to oxy-
gen and water vapor in the atmosphere is usual phe- The surface modified by chemical and physical relax-
nomena taking place everywhere. Also on the surface ations as described above makes surface energy lower
of nitrides and fluorides, atmospheric oxygen adsorbs in the atmosphere, and hence a variety of physical
chemically and then oxides are formed. Chemical adsorption phenomena takes place. On a polar sur-
adsorption of water vapor takes place on the surface of face, physical adsorption takes place and surface free
the oxides, and then hydroxyl groups are eventually energy decreases. On a hydrophilic surface, adsorp-
formed. This is the chemical surface relaxation. tion layer of around two molecules are formed at a rel-
Examples of the chemical adsorption of oxygen on ative humidity less than about 60 %.
the surface of metals, nitrides and fluorides are given The adsorbed water makes solid surface stabilized,
as follows:
and gives a strong effect on chemical, mechanical and
electrical properties of the material. It also makes a

2Cu O 2CuO (1.9.1)
2 change in the interparticle adhesion force as a particle
property and gives a strong effect to phenomena of
4AIN 3O 2Al O 2 (1.9.2) coagulation and consolidation [6–8].
2
3
2
The decrease in the surface free energy can be
obtained from an adsorption isotherm [9]. The
Si N 3O 3SiO 4N 2 (1.9.3) adsorption isotherms of water and n-octane with
4
3
2
2
respect to nano-sized silica particles are shown in
In general, chemical adsorption of water vapor takes Fig. 1.9.3. The decrease in the surface free energy due
place easily on the surface of oxides, which is eventu- to the formation of adsorption layers is called as sur-
ally covered with hydroxyl groups. An example for face pressure , and can be related to the Gibbs’
silica is as follows:
adsorption equation as follows:

Si Si
O 2 Si
(1.9.4) Γ
2

Γ
() RT ∫ Γd ln( p p 0 ) (1.9.7)
For chemical adsorption of water vapor, there is a case Γ=0
to adsorb only on the surface layer and another case to
adsorb on layers extending to the inside of the solid. It where is the surface pressure, R the gas constant,
is known that for MgO the chemical adsorption takes T the temperature, the absorbed amount, p the
place not only on the surface but also on the inside and equilibrium pressure and p the saturated vapor
0
hence layers of magnesium hydroxide are formed. pressure.
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