Page 102 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 2 STRUCTURAL CONTROL OF NANOPARTICLES

50nm 20nm 50nm
(a) [Ba(AOT) ] = 0.01 mol/l (b) [Ba(AOT) ] = 0.01 mol/l (c) [Ba(AOT) 2 ] = 0.1 mol/l
2
2
[Na CrO ] = 0.08 mol/l [Na CrO ] = 0.2 mol/l [Na CrO ] = 0.02 mol/l
2
4
2
4
2
4
2-
2-
[Ba 2+ ] : [CrO ] = 1 : 4 [Ba 2+ ] : [CrO ] = 1 : 10 [Ba 2+ ] : [CrO ] = 10 : 1
2-
4
4
4
Figure 2.3.10
Effect of raw material concentration on product morphology.

50nm 20nm 20nm
(a) [EtOH] = 25vol% (b) [EtOH] = 50vol% (c) [EtOH] = 75vol%
Figure 2.3.11
Effect of poor solvent on product morphology.

AOT molecules selectively adsorb on specific crystal product remained that of a bundle of nanowires
faces along the a axis of the generated particles and (Fig. 2.3.10(c)). Briefly, it is assumed that the gener-
suppress the growth of the adsorbed AOT. Thus, the ated particle is monodisperse, the total volume of
generated particle grows only in the a axis direction generated particles is V, the number of generated par-
and forms a nanowire. Then, the nanowires are self- ticles is N, and the (100) face area of a particles is A.
organized by the hydrophobic interaction between the The length L of a particle along the a axis can be
surface-anchored AOT molecule chains, and a bundle expressed as L V/NA. Since the growth of the crys-
of nanowires is formed. tal face with absorbed surfactant AOT is suppressed,
Figure 2.3.10 shows TEM photographs of the prod- the (100) face area A is almost constant. The particle
uct material obtained when the concentration of bar- number N is a function of the supersaturation ratio S
ium or chromate ions is increased from that for the defined in the following equation:
standard conditions (experimental conditions:
2
2
2
2
(a) [Ba ]:[CrO ] 1:4; (b) [Ba ]:[CrO ] 1:10; 2 2 1/2
4
4
2
2
(c) [Ba ]:[CrO ] 10:1). With an increase in the S ([Ba ][CrO ]/ K ) , (2.3.1)
4
sp
4
relative concentration of the chromate ion, the mor-
phology of the product material changed from a bun- where K is the solubility product of BaCrO . Since
sp
4
dle of nanowires (Fig. 2.3.9) to a chain of nanorods Ba(AOT) has a poor solubility in water and NaAOT
2
(Fig. 2.3.10(a)) and then to dispersed nanodots exists in excess at the interface of the emulsion, the
(Fig. 2.3.10(b)). The aspect ratio of the generated AOT ion concentration always remains saturated
particles decreased with the increase in the relative and constant within the water-filled pools as
concentration of chromate ion. nanometer-sized reaction fields, the free barium ion
In contrast, regardless of any relative increases in concentration in equation (2.3.1) always seems to be
the barium ion concentration, the morphology of the constant at a low concentration. Thus, the number of
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